5w50 vs 5w40 engine oil

What is the difference between 5W40 and 5W50?

5W50 engine oils are specialized, have a high viscosity index close to 200, which means that the oil is very stable under high loads. Oils with such an index have a very high cost, and therefore are not very popular, but are indispensable for very loaded and highly accelerated engines, they are considered sports oils. They are always represented by synthetics.

5W40 oils have a slightly lower viscosity and are also mostly synthetic lubricants. The difference between these engine oils is in the high-temperature index, due to which 50w50 will be more stable at high temperatures and loads. The low-temperature index of the oils is identical, in a cold start, they will show themselves approximately the same.

To compare the difference between the oils, we can give the standard values ​​of the kinematic viscosity of these oils at 100 ℃ (212 °F): for 5W-40 in the range of 12.5-16.3 mm2 / s, for 5W-50 – 16.3-21.9. So, oil with an index of 50 will be ten times thicker.

Comparison of laboratory tests 5W40 and 5W50

It is best to track the difference using a specific example, in this case, I will give the results of laboratory analyzes of Motul 5W-50 and Motul 5W-40 oils.

Kinematic viscosity at 40℃ (104 °F)82.2
Kinematic viscosity at 100℃ (212 °F)13.5
Flash point230℃
Freezing point-36℃

The table clearly shows the difference between the viscosity of oils, even at operating temperatures, the viscosity is much higher, which means it will create a very strong film suitable for extreme conditions. The flash point is also much higher and the overload stability is increased. 

As for the start in cold weather, as I said, the difference will not be big, it all depends on the manufacturer and the composition. In this example, there are small amounts of esters in 5W50 oil, which means that it will be easier to turn the crankshaft on a cold one.

When should 5w50 be used in an engine?

The 5W50 is a good option for a high mileage car, especially since the tolerances are consistent because 5w-50 has a higher viscosity and creates a thicker oil film to protect the engine.

At the same time, 5W50 oils should not be added into engines where manufacturers recommend low-viscosity oils, or into engines with low mileage. Manufacturers design the engine with specific clearances and therefore recommend an oil with the appropriate viscosity to create the optimal oil film thickness. 

If the film is too thick for the engine, then the oil will simply burn off on the cylinder walls. It can also lead to overheating and coking of the rings, since too thick oil film prevents heat dissipation. Among other things, do not forget about the effect of oil on fuel consumption – with an increase in viscosity, energy consumption increases, and, thus, fuel consumption will also increase.

5W40 or 5W50: which is better to use?

Depending on the conditions of use. For normal driving in standard motors, 5W40 will show itself perfectly. For sports driving, 50W50 is better. But we always rely on the recommendations of the engine manufacturer. Increasing the viscosity is always risky, before deciding on this step, deal with all the nuances. When you understand why you need to increase the viscosity, there will be no question “which is better”.

5W40 or 5W50: which is better for winter?

Depends on the composition of the oil, in the example given, Motul 50ka oils will be slightly better, but not significantly. This is true with almost all oils. The difference during cold start on one and the other oil will be almost imperceptible. Basically, they differ in performance precisely at high temperatures and loads.

Can 5W40 and 5W50 be mixed?

It is possible to mix engine oils, but in rare cases and under certain conditions, using the oil of the same brand and without overusing such mixtures. It should be understood that the viscosity of the oil will drop or rise, depending on the amount of added oil. 

If you mix the indicated viscosities 50% – 50%, the result is an average value of 5W-45. We also take into account the composition of the oil – synthetics, semi-synthetics, or mineral. 


The 5w50 oil has a higher viscosity than 5w40, which means it’s thicker and can perform better under high-load. However, this also means that it’s more resistant to shear, meaning that the engine oil won’t thin out as fast as a lighter weight oil at high temperatures or in higher load situations.

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