How to Choose Engine Oil Taking into Account the SAE Viscosity Grades?

The manufacturer recommends engine oil with SAE 0W-20 viscosity, but service centers fill engine oil of a higher viscosity grade ‒ 5W-40. Can it have an unfavourable impact on the working capacity of the engine?


Viscous-temperature rating of engine oil significantly affects almost all the main characteristics of engines. Power, torque, efficiency, engine life ‒ they all require engine oil of a certain viscosity. The viscosity of engine oil is classified according to the SAE system (Society of Automotive Engineers). This classification specifies the highest possible level of low temperature viscosity, as well as the viscosity range at 100° C. But in order to understand what kind of oil to choose, you need to recollect first what engine oil designations mean.

Designations and Hidden Meaning

The easiest way to assess the performance of a simple ball viscometer is to use oils of the same brand (the first number in each designation is different). In our case, these are oils SAE 0W-40, 5W-40 and 10W-40.

The first number points to the minimum temperature for which the oil is designed. If, for example, the first symbol is a zero, cranking is guaranteed at temperatures up to -35° C, and oil pumpability is ensured even at -40° C. To be more precise, the oil producer guarantees that at these temperatures the viscosity of the product will not exceed the values determined by the SAE classification.

The number after the hyphen stands for high temperatures. It indicates the permissible range of changes in viscosity at 100° C. For example, if it is “20”, it means that the manufacturer promises the values from 5.6 to 9.3 cSt, and for “40” from 12.6 to 16.3 cSt. In addition, the same number specifies the minimum viscosity at 150° C.

Which Viscosity is Better?

It is clear that in frosty weather the starter will not turn the engine over, and the pump cannot pump oil if it is too viscous. The less the first number in the designation, the less the engine wear is during startup. This factor does not affect the operation of the warmed-up engine.

At high temperatures the situation is more complicated. It would seem that the higher the viscosity, the better. On the contrary. If you have an ordinary car and fill its engine with W60 engine oil which is not intended for it at all, you are likely not only to lose power, but also to destroy the engine. But why? After all, viscous oil should better protect the parts from wear. The higher the viscosity, the thicker the oil layer in the bearings and under the piston rings is and, correspondingly, the lower the wear rate is.

However, there is the other side of the coin related to the low thermal conductivity of the oil. After all, the thicker the oil layer, the worse heat is removed from the piston which at the same time begins to overheat and expand. Friction grows as well, so jamming is quite possible.

It should be noted that the second number “works” not only at triple-digit temperatures, but also during the warming of the engine. The higher the viscosity, the greater the friction loss is. And the viscosity depends on the temperature. Once we carried out research on this issue and found out that at room temperature the difference in viscosity between W30 oil and W50 oil is almost twice as big. That is why fuel consumption with more viscous oil during the warm-up will be higher.

Now the main question is: what kind of oil does my engine need? Unfortunately, modern research has shown that when choosing the right oil for a particular engine, compliance with the SAE system is not enough. You need a more precise “adjustment” which depends both on the design of the engine and on the conditions in which it works and the degree of wear.

What Will Happen if…

But why do we need all these arguments if the correct answer has long been known? Use exclusively the oil recommended by the car manufacturer! But the manufacturer, as a rule, tries to please the greatest number of consumers irrespective of the working conditions of the car and its age. The group of quality should be treated with respect: if oil with specification SN is recommended, it means that SM oil cannot be used.

But you can experiment a little with the viscosity within permitted limits. For example, for use at low temperatures the second number in the designation can be a bit less than the instructions recommend. Let it be 30 instead of 40, for instance. It will help to reduce fuel consumption to some extent, because in winter it takes oil more time to warm up than in summer, and fuel consumption with viscous oil will be higher, of course.

The same concerns cars that are used mostly in urban areas. If the engine more often works at moderate speeds, the second number in the oil designation can be a bit less than the recommended one for a vehicle that often travels on highways. Again the reason for that is the relationship between the thickness of the oil film, temperature and friction. Employees of specialized laboratories state that there is an optimum viscosity for each engine and its operating mode, and it reduces mechanical loss.

A Bit of Independent Activity

One question remains unanswered, though. How different are the service properties of oils with the same viscosity but from different manufacturers? It is impossible to answer this question without laboratory research.

In any case, we recommend those who love experimenting with unknown oils to use a special instrument to measure the viscosity of the oil. The results of the experiment will be obvious.

But it is not worth experimenting with the car. In any case, we strongly advise you to listen to what the engine manufacturer recommends, not the oil manufacturer. We have already described in what cases and to what extent it is possible to deviate from their guidelines.

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