Oil viscosity at working temperatures

So what happens once the engine, and with it the engine oil, has warmed up to working temperature? At that point the engine's cooling system kicks in. Roughly speaking, it all happens according to a pattern like this (very simplified): under increased load or rpm, the friction coefficient rises => oil temperature rises => oil viscosity drops => the oil film's thickness decreases => the friction coefficient falls => oil temperature drops (with some help from the cooling system), or, at the very least, its rise slows considerably. The loop closes, and the engine keeps running. But the viscosity and temperature of the engine oil don't stand still through all this — they change dynamically, within certain ranges strictly calculated by the engine's manufacturer.

So, in reality, how well an engine runs depends not on the absolute viscosity value at a given temperature, but on how that value changes dynamically while running across a given range of working temperatures, and on how well that dynamic matches the design of the specific engine.

It's worth remembering that any engine, especially a modern one, is a very precise mechanism, and it's largely that precision that determines all the parameters we normally use to judge an engine's appeal to the consumer: power, torque, fuel economy.

And this is exactly where the key question gains special weight: is there actually a difference in clearances and working temperatures between engines of different types, displacements and makers? Yes, there is, and the difference is very substantial, especially with the latest engine models. That's precisely why different carmaker approvals exist for engine oils, as well as different quality classes with different temperature-viscosity requirements under various international classifications (the clearest example being the ACEA classification).

I'll stress that this is by no means only about oils with a different SAE viscosity index! The high-temperature SAE viscosity index is assigned based on the absolute viscosity values of an oil at 100 and 150 °C (see the oil viscosity table for more detail — it has all the ranges). But before, between and after those intermediate reference values, the viscosity-change curve of different oils as temperature shifts can vary quite considerably. Not to mention that even at those specified reference temperature points, the SAE requirements don't call for exact viscosity values but a fairly wide range of them.

So, even two different oils whose labels both say, say, 5W-40, can quite easily have different absolute viscosity at 90, 120 or 145 °C. And it's precisely this dynamic, among other parameters, that's encoded in those same mysterious letters and numbers of carmaker approvals and engine oil quality classifications. And it's worth stressing once again: the dynamic of an oil's viscosity can't be good or bad in itself — it has to be the right fit, that is, matching the design of the specific engine!

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