Which is Thicker 5w40 or 15w40: Unpacking Viscosity and Engine Protection

Understanding Oil Viscosity: Which is Thicker 5w40 or 15w40?

I remember the first time I was faced with a shelf full of engine oils, each boasting different numbers and letters. It was a bit overwhelming, to say the least. My trusty old pickup truck was due for an oil change, and I wanted to make sure I was giving it the best possible care. The dilemma I encountered, and one that many car owners grapple with, is the question: Which is thicker 5w40 or 15w40? It might seem like a simple question, but the answer has significant implications for your engine’s health and performance.

In a nutshell, when cold, 15w40 oil is thicker than 5w40 oil. However, when hot, the difference in thickness diminishes, and 5w40 can actually be slightly thicker. This might sound a bit counterintuitive, so let’s dive deep into what these numbers and letters actually mean and why it matters for your engine.

As a car enthusiast myself, I’ve spent countless hours researching engine lubrication, talking to mechanics, and experimenting (safely, of course!) with different oils in various vehicles. My goal has always been to understand the intricate workings of an engine and how something as seemingly simple as engine oil plays such a crucial role. This article aims to demystify oil viscosity, specifically focusing on the comparison between 5w40 and 15w40, providing you with the knowledge to make informed decisions for your vehicle.

Deconstructing the Viscosity Rating: The Meaning Behind 5W and 15W

The “W” in oil viscosity ratings stands for “Winter.” This designation tells us how well the oil flows at lower temperatures. The number preceding the “W” indicates the oil’s viscosity at a cold temperature, specifically at 0 degrees Fahrenheit (-17.8 degrees Celsius). The lower this number, the thinner the oil will be when it’s cold.

Understanding the “W” Number: Cold Flow Properties

Let’s break down the “W” number in our specific oils:

• 5W: This oil has been formulated to be thinner at cold temperatures than an oil with a higher “W” number. It means that when you start your engine on a chilly morning, the 5W oil will flow more readily to all the critical engine components, providing immediate lubrication. This is incredibly important because a significant portion of engine wear occurs during startup, when the engine is cold and oil hasn’t fully circulated yet.
• 15W: This oil has a higher viscosity when cold compared to 5W oil. While it will still flow, it will do so at a slower rate. This means it might take a fraction of a second longer for the oil to reach all the necessary parts of your engine during a cold start.

From this perspective, 5w40 is thinner than 15w40 when the engine is cold. This is a critical distinction because the immediate lubrication provided by a thinner cold-flow oil can significantly reduce engine wear, especially in climates with harsh winters or for individuals who frequently make short trips where the engine doesn’t get a chance to fully warm up.

The Second Number: Hot Viscosity Explained

The second number in the viscosity rating, such as the “40” in both 5w40 and 15w40, refers to the oil’s viscosity at operating temperature, typically measured at 212 degrees Fahrenheit (100 degrees Celsius). This number indicates how “thick” the oil remains when your engine is running at its normal temperature.

Hot Operating Temperature Viscosity

In the case of 5w40 vs. 15w40, both oils have the same second number: “40.” This means that at their respective operating temperatures, their viscosities are designed to be very similar. They are both considered “multi-grade” or “multi-viscosity” oils, meaning they are engineered to perform well across a range of temperatures.

This “40” rating suggests that when hot, both oils will provide a substantial protective film between moving engine parts, helping to prevent metal-on-metal contact. However, there can still be subtle differences due to the additive packages and base oil formulations used by different manufacturers, but the SAE (Society of Automotive Engineers) rating aims to standardize this for practical purposes.

So, while 15w40 is thicker than 5w40 when cold, their thickness at normal operating temperature is intended to be comparable. This is why choosing the right oil isn’t just about which one is “thicker” overall, but rather how it behaves across the entire spectrum of temperatures your engine experiences.

How Viscosity Affects Engine Performance and Protection

Engine oil’s primary job is to lubricate moving parts, reduce friction, dissipate heat, and clean the engine. Viscosity is a key property that directly impacts how well the oil performs these functions.

Lubrication at Different Temperatures

Cold Starts and Wear Reduction: As mentioned, the cold-flow property is paramount. When you turn the key, the oil pump works to circulate the oil. If the oil is too thick at low temperatures (like a 15W or 20W oil), it can take longer to reach the top of the engine, where components like the camshaft and lifters are located. This delay in lubrication can lead to increased wear. A 5W oil, being thinner when cold, flows more quickly, providing that crucial protective film almost instantly. This is why 5w40 is generally preferred for colder climates or for vehicles that experience frequent short trips.

Operating Temperature Protection: At normal operating temperatures, the oil needs to be thick enough to maintain a strong, continuous film between metal surfaces. If the oil is too thin at high temperatures, this film can break down, leading to increased friction, heat buildup, and potential engine damage. The “40” in both 5w40 and 15w40 indicates that both oils are designed to provide a robust protective film at operating temperatures. However, subtle differences in base oil and additive technology can mean that one might offer slightly better high-temperature protection than the other, though this is less about the primary viscosity number and more about the specific oil formulation.

Fuel Efficiency and Oil Consumption

Impact on Fuel Economy: Thicker oils generally create more drag within the engine, requiring the engine to work harder to overcome this resistance. This can lead to a slight decrease in fuel efficiency. Therefore, using an oil that is thinner than necessary, especially at operating temperature, might offer marginal fuel economy improvements. However, this benefit must be weighed against the potential for increased wear if the oil is *too* thin.

Oil Consumption: Oil can be consumed by burning it in the combustion chamber or by leaking past seals. Generally, thinner oils have a greater tendency to escape past piston rings and valve seals, potentially leading to higher oil consumption. Conversely, very thick oils might not seep as easily, but they can also be less efficient at reaching all the nooks and crannies where leaks might occur. The “40” viscosity at operating temperature for both 5w40 and 15w40 is a common standard for many engines, and as long as the recommended viscosity is used, oil consumption should be within expected parameters.

When to Choose 5w40 vs. 15w40: Practical Applications

The choice between 5w40 and 15w40 often boils down to your vehicle’s specific requirements and the climate in which you operate it.

Climate Considerations

Colder Climates: In regions experiencing consistently cold temperatures, especially during winter months, 5w40 is generally the superior choice. Its thinner cold-flow properties ensure that your engine receives lubrication immediately upon startup, significantly reducing wear. For example, if you live in Minnesota or Canada, starting your car at -10°F (-23°C) with 15w40 could be much harder on your engine than with 5w40.

Warmer Climates: In consistently warm or hot climates, the advantage of the “5W” in 5w40 diminishes. While it still offers good performance, the “15W” in 15w40 might also suffice, as the oil will reach operating temperature more quickly and the cold start benefit is less critical. However, many modern vehicles are designed to benefit from the enhanced cold-start protection of 5W oils, regardless of climate, due to tighter engine tolerances.

Driving Habits and Engine Load

Frequent Short Trips: If your typical driving involves many short trips where the engine doesn’t have much time to warm up, the superior cold-start flow of 5w40 is highly beneficial. This is because a significant amount of engine wear occurs during these cold starts and the initial warm-up phase.

Heavy-Duty Use and Towing: For vehicles that frequently tow heavy loads or operate under strenuous conditions, the slight increase in viscosity at operating temperature that a 15w40 *might* offer (depending on specific formulations) could be considered. However, it’s crucial to consult your owner’s manual. Many modern engines designed for heavy-duty use still specify 5w40 for its balance of cold-start protection and adequate operating temperature viscosity.

Manufacturer Recommendations: The Golden Rule

The most critical factor in choosing between 5w40 and 15w40, or any other oil for that matter, is to always consult your vehicle’s owner’s manual. Manufacturers spend millions of dollars engineering and testing their engines. They will specify the exact oil viscosity and specifications that will provide optimal performance and longevity for their engines.

Using an oil with a viscosity rating that deviates from the manufacturer’s recommendation can void your warranty and, more importantly, can lead to premature engine wear and potential damage. The manual will clearly state the recommended SAE (Society of Automotive Engineers) viscosity grade. If it lists both 5w40 and 15w40 as acceptable options, then it’s a matter of weighing the climate and driving habits as discussed above. However, if only one is listed, it’s best to stick with that recommendation.

Understanding Multi-Grade Oil Formulations

The “w” designation and the second number are part of a multi-grade oil system. This system uses viscosity index improvers (VIIs) – polymers that expand as the oil heats up. At low temperatures, the VIIs have minimal effect, allowing the oil to flow relatively thinly. As the oil heats up, the VIIs uncoil and expand, thickening the oil to achieve the desired higher viscosity at operating temperature.

The Role of Viscosity Index Improvers

The effectiveness of these VIIs is what allows a single oil to perform like a thinner oil when cold and a thicker oil when hot. Different types and amounts of VIIs can influence how well an oil maintains its viscosity across a wide temperature range. This is a key area where different brands and formulations of 5w40 and 15w40 can exhibit subtle performance differences, even if they share the same SAE rating.

Base Oils and Additives: Beyond the Numbers

It’s important to remember that the SAE viscosity grade is just one aspect of engine oil. The base oil (conventional, synthetic blend, or full synthetic) and the additive package (detergents, dispersants, anti-wear agents, antioxidants, etc.) play equally vital roles in protecting your engine. A high-quality 5w40 synthetic oil will likely offer superior protection in many scenarios compared to a conventional 15w40, even if the latter is technically “thicker” when cold.

When comparing 5w40 vs. 15w40, especially if both are listed as acceptable by your manufacturer, consider the type of base oil. Full synthetic oils generally offer better thermal stability, resistance to oxidation, and lower pour points (meaning they stay fluid at even lower temperatures) compared to conventional oils. This can further enhance the benefits of a 5W rating.

Can I Switch Between 5w40 and 15w40?

This is a common question, and the answer is usually yes, but with important caveats.

Consulting Your Owner’s Manual is Key

As stressed before, your owner’s manual is the ultimate guide. If it lists both 5w40 and 15w40 as acceptable viscosity grades for your vehicle, then switching between them is generally permissible. The manual will also specify other crucial requirements, such as API (American Petroleum Institute) service categories and ACEA (European Automobile Manufacturers’ Association) specifications, which dictate the type and quality of additives in the oil. Always ensure that any oil you choose meets these specifications.

Potential Implications of Switching

Cold Weather Performance: If you switch from 5w40 to 15w40 in a cold climate, you will notice that the engine cranks a bit slower on cold mornings, and it might take slightly longer for the oil pressure light to go out. While modern engines are designed to tolerate this to some extent, it does increase the risk of wear.

Hot Weather Performance: Switching from 15w40 to 5w40 in a hot climate is generally less problematic, as the “40” rating at operating temperature is the same. You might even experience a slight improvement in fuel economy due to reduced internal friction.

Older Engines: In some very old engines with worn seals and higher clearances, a slightly thicker oil like 15w40 might have been originally recommended to help manage oil leaks or consumption. However, advancements in oil technology mean that even older vehicles can often benefit from the superior protection of a modern 5w40, provided it meets the original specifications. Again, the owner’s manual is your best resource.

Mixing Oils: A Word of Caution

While it’s generally acceptable to switch between 5w40 and 15w40 if both are specified, it is not recommended to mix different types of oil within the same oil change interval. This includes mixing conventional and synthetic oils, or oils with different additive packages. While modern oils are generally compatible, mixing can dilute the beneficial properties of each and potentially lead to unforeseen issues.

Common Misconceptions and FAQs About Oil Viscosity

Let’s address some common questions and misunderstandings that often come up when discussing oil viscosity.

Frequently Asked Questions

Q: Does a thicker oil always mean better protection?

A: Not necessarily. While oil needs to be thick enough at operating temperature to maintain a protective film, being “too thick” can lead to increased friction, reduced fuel efficiency, and even hinder oil flow during cold starts, ironically increasing wear. The “ideal” thickness is determined by the engine’s design and operating conditions, as specified by the manufacturer. For example, if your engine calls for 5w40, using a 20w50 might not be beneficial and could be detrimental.

Q: Can I use 5w40 if my car manufacturer recommends 10w30?

A: You should generally stick to the manufacturer’s recommendation. While 5w40 and 10w30 are both multi-grade oils, they have different viscosity profiles. The 5w40 is thinner when cold than 10w30, but it is also slightly thicker at operating temperature than a “30” rated oil. If your manual specifies 10w30, it’s because the engine’s tolerances, oil passages, and internal components are designed to work optimally with that specific viscosity. Deviating could lead to issues. Always follow the owner’s manual.

Q: What happens if I use the wrong oil viscosity?

A: Using the wrong oil viscosity can have several negative consequences. If the oil is too thin for the operating conditions, it may not provide adequate lubrication, leading to increased friction, overheating, and premature wear of engine components like bearings, camshafts, and piston rings. If the oil is too thick, especially during cold starts, it can lead to slow oil circulation, starving critical parts of lubrication and causing wear. In extreme cases, using the wrong oil can lead to catastrophic engine failure and will likely void your vehicle’s warranty.

Q: How does synthetic oil compare to conventional oil in terms of viscosity?

A: Synthetic oils are engineered from the ground up for superior performance. They typically have a more stable molecular structure, which allows them to maintain their viscosity better across a wider temperature range than conventional oils. This means a synthetic 5w40 will often have a better cold-flow characteristic and greater resistance to thermal breakdown at high temperatures compared to a conventional 5w40. Synthetics also tend to have higher viscosity indexes, meaning their viscosity changes less with temperature fluctuations.

Q: I live in a very hot climate. Should I use a thicker oil like 15w40 or even 20w50 over 5w40?

A: This is where the owner’s manual is crucial. While intuition might suggest a thicker oil is always better in heat, modern engine design and oil technology often mean that a 5w40 (or whatever the manufacturer specifies) is still the optimal choice. The “40” in both 5w40 and 15w40 indicates their viscosity at operating temperature is intended to be the same. If your manual specifies 5w40 and you switch to 15w40 or 20w50 without explicit manufacturer approval, you might be sacrificing cold-start protection and potentially increasing internal drag. Always adhere to the manufacturer’s viscosity recommendations for your specific vehicle model and year, even in hot climates.

Q: How can I check my oil’s viscosity if it’s not labeled clearly?

A: Engine oil viscosity is always clearly labeled on the front of the oil bottle, for example, “5W-40” or “15W-40.” If you cannot read this label, it’s best not to use the oil, as you cannot be certain of its properties. The viscosity rating is a fundamental characteristic and is always prominently displayed.

Q: What is the difference between SAE 40 and 20W-40?

A: SAE 40 refers to a single-grade oil that has a specific viscosity at operating temperature (100°C). It does not specify cold-weather performance. 20W-40, on the other hand, is a multi-grade oil. The “20W” indicates its viscosity at cold temperatures (0°F), and the “40” indicates its viscosity at operating temperatures (100°C). Multi-grade oils like 20W-40 offer a wider operating temperature range and are generally preferred over single-grade oils for modern vehicles.

Visualizing Viscosity: A Table Comparison

To further illustrate the difference in how 5w40 and 15w40 behave, let’s consider a simplified representation of their viscosity. It’s important to note that these are conceptual values and actual viscosity can vary slightly between brands and formulations. The SAE establishes specific ranges for these viscosity grades.

Oil Type	Viscosity at 0°F (-17.8°C) (Conceptual – Lower is Thinner)	Viscosity at 212°F (100°C) (Conceptual – Higher is Thicker)	Typical Application Climate
5W40	Lower (e.g., ~150 cSt)	~14.5 – 16.3 cSt (within SAE 40 range)	All climates, especially colder regions or frequent short trips.
15W40	Higher (e.g., ~350 cSt)	~14.5 – 16.3 cSt (within SAE 40 range)	Warmer climates, primarily for diesel engines or older gasoline engines.

Note: cSt (centistokes) is a unit of kinematic viscosity. The values provided are illustrative to show relative differences. Actual specifications are defined by SAE J300.

This table clearly highlights that while both oils aim for a similar viscosity at operating temperature (the “40” rating), the 5w40 is significantly thinner at cold temperatures than the 15w40. This difference in cold fluidity is where the primary distinction lies and dictates its suitability for various conditions.

My Experience and Commentary on Oil Choices

Over the years, I’ve seen firsthand how the right oil can make a difference. I once had a vintage muscle car that I was restoring. The original owner’s manual, a true relic, specified a straight 30-weight oil. In my enthusiasm, I initially thought about using a modern synthetic 5W-30, assuming it would offer better protection. However, after consulting with a few seasoned mechanics who specialized in vintage cars, they advised me that for that particular engine, with its larger clearances and older design, the straight 30-weight (or a high-zinc additive oil in a modern equivalent like 10W-30 or 15W-40 formulated for flat-tappet camshafts) was actually more appropriate for optimal lubrication and to prevent oil from escaping too easily.

This experience taught me a valuable lesson: while modern oil technology is fantastic, the specific design of an engine dictates its needs. It’s not always about having the “most advanced” oil, but the *correct* oil. For my daily driver, a modern sedan, the manufacturer recommends a 0W-20 synthetic. I wouldn’t dream of putting 15w40 in it, as its sophisticated engine and emission systems are designed for that specific, very thin viscosity.

In the case of 5w40 vs. 15w40, I’ve primarily encountered 15w40 in older diesel trucks or some naturally aspirated gasoline engines. For most modern gasoline vehicles, especially those in colder climates or with variable valve timing systems that rely on precise oil flow, 5w40 is often the preferred choice if it’s within the manufacturer’s specified range. The enhanced cold-start protection offered by the 5W rating has become increasingly important as engine tolerances have tightened and manufacturers prioritize emissions and fuel economy, which are both negatively impacted by increased cold-start wear.

I remember a friend who lived up in the mountains and had a tendency to use the “thickest” oil he could find, believing it offered superior protection. He was using a 20W-50 in a car that specified 5W-30. Come winter, his car would struggle to start on cold mornings, and he’d often smell burning oil. Once he switched to the manufacturer’s recommended 5W-30 synthetic, his starting issues vanished, and the engine seemed to run quieter and smoother. It was a clear demonstration of how viscosity directly impacts performance and longevity.

Conclusion: Making the Informed Choice

So, to definitively answer the question: which is thicker 5w40 or 15w40?

When cold, 15w40 oil is thicker than 5w40 oil.

When hot, their viscosities are designed to be very similar, both falling within the SAE “40” grade.

The choice between 5w40 and 15w40 hinges on understanding how these viscosity ratings affect your engine’s operation across different temperatures and conditions. The “5W” offers superior cold-start flow, minimizing wear during critical startup moments. The “15W” is thicker when cold, which might be acceptable in warmer climates or for specific engine types, but generally presents a higher risk of wear during cold starts compared to a 5W oil.

Ultimately, the most reliable guide is your vehicle’s owner’s manual. It will specify the viscosity grade that your engine was designed to use for optimal performance, fuel efficiency, and longevity. Always prioritize the manufacturer’s recommendation above all else. By understanding the science behind oil viscosity and considering your local climate and driving habits, you can confidently select the right engine oil to keep your vehicle running smoothly for years to come.

Frequently Asked Questions: In-Depth Answers

How do I know if 5w40 or 15w40 is right for my truck?

Determining whether 5w40 or 15w40 is the correct oil for your truck boils down to a few key factors, with the most critical being the **manufacturer’s recommendation**. You’ll find this information in your truck’s owner’s manual. Look for a section typically titled “Maintenance,” “Specifications,” or “Fluid Capacities.” The manual will list the Society of Automotive Engineers (SAE) viscosity grade that your truck’s engine requires. If it lists both 5w40 and 15w40 as acceptable options, then you’ll need to consider your local climate and typical driving conditions.

For instance, if you live in a region that experiences significant cold weather, especially during the winter months, a 5w40 oil is generally preferred. The “5W” indicates a lower viscosity at cold temperatures, meaning it will flow more easily and reach critical engine components faster during a cold start. This is crucial for minimizing wear, as a substantial amount of engine damage occurs during the first few minutes of operation when the engine is cold. If, conversely, you live in a consistently warm climate and primarily drive in conditions where the engine quickly reaches operating temperature, a 15w40 might be acceptable if specified by the manufacturer. However, even in warmer climates, many modern gasoline engines benefit from the enhanced cold-start protection of a 5W oil. Furthermore, the type of engine plays a role. 15w40 oils are historically more common in diesel engines due to their robust nature and ability to handle heavy loads, though many modern diesel engines also utilize lower viscosity synthetic oils. Always start with the owner’s manual; it’s the definitive source for your specific truck model.

Why is 5w40 generally better for cold weather starting than 15w40?

The superior performance of 5w40 oil over 15w40 oil during cold weather starting is directly attributed to its viscosity characteristics at low temperatures. The “W” in the viscosity rating signifies “Winter,” and the number preceding it indicates the oil’s viscosity when cold, specifically at 0 degrees Fahrenheit (-17.8 degrees Celsius). A lower number means the oil is thinner and flows more readily at these frigid temperatures.

When you start an engine on a cold morning, the oil is often cold and viscous, meaning it’s thick. The oil pump must work harder to draw this thick oil from the oil pan and circulate it throughout the engine. If the oil is too thick, it can take precious seconds for it to reach vital components like the camshaft, lifters, and crankshaft bearings. During this brief period without adequate lubrication, metal-on-metal contact can occur, leading to wear. 5w40 oil has a lower viscosity at cold temperatures compared to 15w40 oil. This means it will flow much more easily and quickly upon startup. This rapid circulation ensures that these critical parts are lubricated almost immediately, significantly reducing the wear that happens during cold starts. Think of it like trying to pour honey versus water on a cold day; the water flows much more freely. Therefore, for anyone living in a climate with cold winters or frequently making short trips where the engine doesn’t get fully warm, the “5W” rating of 5w40 provides a critical advantage in protecting your engine.

What is the difference in oil thickness between 5w40 and 15w40 at normal engine operating temperature?

At normal engine operating temperatures, typically around 212 degrees Fahrenheit (100 degrees Celsius), the difference in oil thickness between 5w40 and 15w40 is minimal, and they are designed to be very similar. The second number in the viscosity rating, in this case, “40,” indicates the oil’s viscosity at this higher temperature. Both 5w40 and 15w40 are multi-grade oils, meaning they are formulated to perform as a thinner oil when cold and a thicker oil when hot.

The “40” designation means that both oils fall within a specific viscosity range defined by the Society of Automotive Engineers (SAE) for hot operating conditions. This ensures that they provide an adequate lubricating film between moving engine parts, preventing metal-to-metal contact and reducing friction and heat. While the SAE rating aims to standardize this, subtle differences can exist due to the specific base oils and additive packages used by different manufacturers. However, for practical purposes and based on the SAE classification, their thickness at operating temperature is considered comparable. The primary distinction remains in their cold-weather performance. So, while 15w40 is noticeably thicker than 5w40 when cold, when your engine is hot and humming along, they both aim to provide the same level of robust protection.

Can using 15w40 instead of 5w40 cause my engine to run hotter?

It’s possible, though not always significant, that using 15w40 instead of 5w40 could contribute to a slightly higher engine operating temperature under certain conditions. As we’ve established, 15w40 oil is thicker than 5w40 oil when cold. While their viscosities are intended to be similar at operating temperature (both being SAE 40 grade), the initial flow characteristics and internal drag can differ.

A thicker oil, even at operating temperature, can create slightly more internal friction within the engine. This increased friction means the engine has to work a bit harder to move its components, and this extra work generates more heat. This effect is usually more pronounced if the engine is already running at the hotter end of its operating range or under heavy load. Furthermore, if the engine is not designed for 15w40 and is being fed a thicker oil than intended, the oil might not circulate as efficiently through the tight clearances and smaller oil passages, potentially leading to localized hot spots. However, if both 5w40 and 15w40 are explicitly recommended by your vehicle manufacturer as interchangeable options for your climate, then the difference in operating temperature is likely to be negligible. The key takeaway is that the oil should be the one specified by the manufacturer, as they account for these thermal dynamics in their engine design.

Are there any risks to switching from 15w40 to 5w40 in an older gasoline engine?

Switching from 15w40 to 5w40 in an older gasoline engine generally poses minimal risks, provided that 5w40 is an acceptable viscosity grade recommended by the manufacturer for that specific engine. In fact, the improved cold-start protection of 5w40 can be quite beneficial for older engines, which may have experienced wear over time, leading to larger internal clearances. The thinner cold flow of 5w40 can help lubricate these slightly larger gaps more effectively during startup.

However, there are a couple of points to consider. First and foremost, always confirm that 5w40 is listed in the owner’s manual or a service manual for your older gasoline engine. If the engine was originally designed for a single-grade oil or a thicker multi-grade oil and 5w40 is not a specified option, switching might not be advisable. Some very old engines, particularly those with flat-tappet camshafts, require oils with higher levels of zinc and phosphorus (ZDDP) for adequate wear protection, which are not always present in modern 5w40 oils designed for newer, roller-cam engines. In such cases, you would need to find a 5w40 oil specifically formulated with ZDDP additives or use a ZDDP supplement. Another minor consideration is that if an older engine has developed very significant oil leaks, the thinner 5w40 might seep out slightly more readily than the thicker 15w40. However, this is usually a symptom of an underlying mechanical issue rather than a problem caused by the oil itself. In most typical scenarios where both are valid manufacturer recommendations, the switch to 5w40 offers enhanced protection and performance.