15w40 vs. 15w50 Engine Oil: What Are The Differences?

15w40 vs 15w50 Engine Oil

Engine oil is necessary for both the automobile industry and the machinery industry. You should use quality engine oil to prevent wear and tear on the engine’s moving parts. These lubricants can also help your engine run smoothly. If you use quality engine oil, it will last for a long time.

In addition, by circulating process, this Oil helps keep your engine from overheating. Knowing about different engine oils and how they differ is very important. We will focus on 15w40 vs. 15w50 engine oil in this article. We also compare them to get a better understanding, so let’s get started!

15w40 vs 15w50 Engine Oil: Differences

Mobil 1 15w50 and Rotella T4 15w40 next to each other to introduce the 15w50 vs 15w40 conversation

Engine oil needs to lubricate the engine’s internal combustion. During running, your car’s moving parts will wear out, so you should lubricate your engine with engine oils. However, let’s compare 15w40 vs. 15w50 engine oil.


Adequately, 15w50 engine oil has a slightly higher viscosity than 15w40 Oil. 15w50 fully synthetic engine oil provides you with excellent protection and lubrication. Also, this 15w50 engine oil is well known for its long drain intervals. So, after traveling about 690 km, you can change this Oil.

Additionally, 15w40 engine oil is best for diesel engines in hot weather. 15w50 contains a higher viscosity rate than 15w40, so it may not be best for diesel engines (depending on the circumstances). Low and high-temperature oil viscosities can be the same for both types of Oil.

Fuel Economy

Accordingly, 15w40 engine oil suits brand-new vehicles in cold weather. But to get the best gas mileage, though, it’s best to use the manufacturer’s engine oils. Engines that need a high-performance level should use 15w50. Even if your machine is overworking and racing in hot temperatures, you should stick to this 15w50.

Yet, you also can use this 15w50 in your turbocharged engines. The fuel economy is similar between them. To benefit from increased fuel efficiency, you must use the correct Oil.

Weather Condition

You can use 15w40 in hot and cold weather without any problems. If you see the motor oil label is on “15”, it doesn’t suggest that temperature will always be limited to 25°F.

There is a direct relation between a low first and low second number. Better to use thin Oil in winter. So using the 15w40 engine oil is best for winter. In contrast, 15w50 engine oil is suitable for cold and hot conditions.

Difference Between 15w40 and 20w40

Oil viscosity is measured at two temperatures because all oils become thinner as they heat. That’s why the multigrade oils are designed not to run as thin as single-grade oils.

20w40 engine oil, meaning the first number tells you its range of viscosities when the Oil is cold. And the 2nd number informs you of its range of viscosities at the upper temperature of 100c.

The ‘W’ in the middle of the number tells you you are “cold.” That means the more waxy material was removed to be liquid at low temperatures.

Likewise, A 20w40 tells you that it is slightly more viscous at cold temperatures than a 15w40. But it’s thin as a 40 at high temperatures. This feature makes 15w40 a good choice if you live in a cold area.

Difference between SAE 40 and 15w40:

There are many differences between SAE 40 and 15w40. An engine oil’s operating temperature doesn’t vary considerably between various ambient temperatures. For that reason, the recommended viscosity is relatively compatible with several applications. Mainly the main difference is the viscosity grade related to the starting temperature.

This can determine the lubricant’s viscosity and readiness to lubricate the engine. A 15w viscosity grade oil is best to use at a lower starting temperature.

In tropical climates, most OEMs still recommend a multigrade product such as SAE 15W40. The lubricant flows through the engine faster than SAE 40 at high ambient temperatures. Generally, SAE 40 Oils are good for mono-grade recommended lubrication, such as gas engine oil.

Which Is Better, 15w40 vs 15w50 Oil?

15w40 engine oil is recommended for heavy-duty engines in hot and humid conditions. Before using 15w50, you should check your owner’s manual because your machine may not be able to handle this Oil.

In cold weather, this Oil also reduces horsepower. Consequently, it would help if you consider your surroundings. Another factor to consider is engine temperature. Nonetheless, it’s best to use any oil recommended by your vehicle manufacturer.

15w50 vs 10w40:

Mobil 1 15w50 and Rotella Valvoline conventional 10w40 motor oil next to each other with a "versus" sign in the middle

A 10w40 grade oil will retain its performance up to 40°C or 104°F. On the contrary, a 15w50 grade oil will maintain its performance up to 50°C or 122°F.

The number before the “W” tells you how the Oil will flow when cold. Yet, 10w40 grade oil is best for low temperatures, while 15w50 Oil is best for high temperatures. Typically, 10W40 is as thicker as most engine manufacturers recommend. On the other hand, 15W50 enhances wear fuel consumption and decreases power.

15w40 vs 10w40:

Both engine oils contain their distinct characteristics. There are also some standard features. And the main difference between 10w40 and 15w40 is the oil temperature range and viscosity. The low-temperature viscosity of 10w40 is ’10’ while for 15w40 is ’15’.

The viscosity index of 15w40 motor oil stands at 135, while for 10w40, it is 150. Therefore viscosity indices are higher in 10w40 Oil than in 15w40. But they both have the same high or operating temperature viscosity rating of 40. Besides this, the viscosity index of 15w40 is 135, while the 10w40 viscosity index is 150. But their high operating temperature viscosity rating is 40.

In cold weather, 10w40 Oil will be thicker than 15w40 Oil. But they contain the same viscosity at high temperatures. So a 15w40 lighter weight engine provides better fuel efficiency in the cold season. Regardless, 10w40 engine oil is more costly than 15w40 engine oil. Usually, 10w40 Oil is better for a moderate engine, but 15w40 engine oil is best for a regular machine.

Check: 20W40 Vs 10W40 Motorcycle Oil

15w40 vs 15w50 Engine Oil
Diesel Engine

FAQ (Frequently Asked Questions)

Q. What problem creates if I replace 15w40 Oil with 15w50 Oil?

The owner’s manual said that using non-recommended Oil can cause considerable damage to your engine’s moving parts. Furthermore, it will reduce your engine’s performance.

Most importantly, 15w40 is hard on your machine and reduces horsepower in colder environments. But if you use lighter Oil in the tropics, you run the risk of fast engine failure. Generally, 15w50 engine oil has better performance than 15w40 engine oil.

Q. Can I use 15w40 Oil on my bike?

You probably can, but it’s not advisable. It only depends on the bike manufacturer’s specifications. Most bikes come with a clutch, so using the wrong Oil can cause a clutch slip. If you have a Honda Super Cub or one of its derivatives, then 15-40 engine oil is fine.

Q. Can you mix 15w40 with 15w50?

No. The reason is that 15w50 Oil contains more viscosity than 15w40. Moreover, this Oil is heavier than 15w40. Heavy Oil can’t contain good temperature protection. If you mix them, they will cause severe damage to the engine.

Q. What does 15w50 Oil Means?

15W (w=winter) implies flowability in cold weather. The Oil is still pumpable down to -25 °C, which means fluid enough to dispense into the engine. The number “50” describes the engine oil flowability of engine oil at a 100°C operating temperature.

Final Word:

In short, each engine requires particular engine oil. Along with this, it’s vital to know your machine’s recommended and proper engine oil. Overall, 15w50 and 15w40 engine oil are suitable for their recommended engine.

The question is 15w40 vs. 15w50: which is better engine oil? We have described above in detail about them. Ultimately, we hope you get an accurate idea of what you want. At last, we want to say both oils come with their advantages and features. But before using any of them, it’s better to read your owner’s manual or consult an expert.

READ: 15W40 VS 20W50


I am Engr. Z Eusuf Lubricant Specialist, Mechanical Engineer & has been working with engine lubricants for a long time and knows what makes them work best. I can help you find the right oil for your needs, and I have tips on how to keep your engine running at its best.