10w40 vs 15w40. The differences between these engine oil types

The discussion about the different engine oils on the market and which of these types is better has been and is always long, full of details, and depends a lot on the engine of the car you are referring to.

The main differences between 10w40 and 15w40 are in the temperature range of the oil and its viscosity at 100° C or 212° F. For example, 10W40 means that it can be used at temperatures not lower than -20° С or -4° F, and 15W40 not lower than -15° С or 5° F, and both engine oils can be used at 40° С or 104° F.

In general, both 10w40 and 15w40 oils are used for engines that have high mileage of 100.000 km (60.000 miles) or more.

10w40 vs 15w40 advantages and disadvantages

Questions regarding which engine oil to choose are of interest to drivers who are advised at the car repair shop to add one of these types of oil into the engine. Any packaging is labeled, so this information will not be in vain for the driver. 

The marking determines the viscosity of the oil and its purpose. Let’s look at the markings and try to understand which engine oil is best to choose for the motor.

The most significant characteristic is the viscosity of the motor oil. It determines how intensively the oil will spread through the engine, as well as how it will change as a result of an increase or decrease in temperature. 

Note that the viscosity class does not indicate the oil’s quality. There are many products of the same viscosity on the market from different manufacturers. 

All of them are different in quality. However, the quality cannot be determined, there are no corresponding indicators. It will be determined in practice.

In the marking of 15W40 oil, we have got 2 parameters: 15W and 40. The first parameter indicates that the oil can be used in winter as well, the letter W means Winter and is intended for use at temperatures not lower than -15° С or 5° F. 

15w40 will work well in these temperatures, it will retain its viscosity, characteristics, and protective properties, which means it will allow the engine to start without any problems, ensuring effective lubrication.

However, at a lower temperature than the one indicated, its viscosity will increase, and it will make the work of the oil pump difficult, it may not even reach the friction parts. This will lead to rapid wear of the engine as a whole.

The number 15 in the title does not at all mean that the lower temperature limit is at -15° С or 5° F. In this case, it’s just a coincidence. 10W40 oil can be operated within the range from -20° С to +40° С or -4° F to 104° F. 

The lower the first digit in the marking, the lower the temperature limit, and the more fluid the oil are. In colder temperatures, this parameter is extremely important, but in the summer it is advisable to use thicker oil, 15W40 should do it.

We should remember that W in the composition of the oil indicates that the oil is universal and can be used in both low and high-temperature environments. Not to be confused with oils that do not have the inscription W and cannot be used at low temperatures.

In general, today universal oils completely replace seasonal oils. It is logical because changing the lubricant every season is not only problematic but also expensive and time-consuming.

The second number in the marking (40) indicates the ambient’s highest temperature limit, at which the oil will not lose its properties. In our example, 15W40 oil can operate in the temperature range from -15° С to +40° С or 5° F to 104° F, without losing its lubricating features.

Which engine oil should you choose?

The best oil for your engine is the one specified in the car’s manual. Manufacturers always indicate the viscosity and even the brand of oil that needs to be added to the engine.

The power of the oil pump, the size of the passage channels, and other features affect the speed of the lubricant in the motor. The wrong choice can become a headache for the driver and cause damage that requires expensive repairs.

Also, when choosing the oil, you need to take into account the climate conditions in which the car will be operated. If temperatures are cold, then the oil should be more liquid and designed to work without any problems in these conditions. 

In this case, maybe neither 10w40 nor 15w40 will suit if the temperatures drop below -20° С or -4° F, you should turn your attention to other oils like 5w30, 5w40, 0w30, 0w40, etc. that support lower temperatures.

It is also worth considering the engine condition. Engine wear plays an important role, with severe wear of the friction parts, the gap between them increases, and adding a liquid oil will make it simply flow away. Therefore, it is recommended to add more viscous oil to older engines.

At the same time, old motors can have carbon deposits and various build-ups in the oil system. For cleaning, drivers are advised to use oils with detergents, which protect the engine and prevent the formation of dirt and sediments.

Also, when choosing a lubricant, you need to consider the driving style. For aggressive driving with sharp starts and high speed, it is advisable to select a more viscous oil, which forms a strong film between friction parts. It will save the engine from the effects of heavy loads.

Also, there are a lot of misconceptions regarding engine oils. Some drivers believe that 10W40 is better than 15W40 oil. Thus, the price of the latter may be higher, or lower. 

This is completely wrong because the viscosity grade is not related to its quality. The viscosity grade does not even determine the base of the lubricant. Any oil, including semi-synthetic, mineral, or synthetic, can have a viscosity grade of 15W40.


As you can see the most conclusive difference between 10w40 and 15w40 engine oils is the low temperature of the environment in which it can be used, this difference being very small between the two, theoretically 5° С or 9° F, but this temperature difference is enough for the engine to suffer serious damage if a wrong oil is used.

Use these two types of oil especially if you have an older engine, but also check the info in your vehicle’s manual, you will find important information there regarding this aspect, especially the ACEA and API standards

Also, between the three types of oil that are on the market, full-synthetic, semi-synthetic also known as synthetic-blend, or mineral, it is always best to use the full-synthetic version, but most of the 10w40 and 15w40 on the market are synthetic-blends.

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