What is oil viscosity? What it's for

The main job of oil is to prevent dry friction between the engine's moving internal parts, while also ensuring minimal friction force with maximum sealing of the working cylinders. It's clear that making a substance with the properties needed for that, and one that also holds stable characteristics across a wide temperature range, is impossible — and the operating temperature range of oil inside an engine is fairly wide.

It's worth noting that the temperature most drivers see on the dashboard, commonly called “engine temperature,” is actually the coolant temperature, which really is stable in a warmed-up engine and should sit around 90 degrees. Oil temperature, meanwhile, swings a lot and can reach 140-150 degrees, depending on speed and how hard the engine is worked.

Based on this, for each individual engine the manufacturer sets a compromise set of optimal oil parameters. These are the parameters that, in the engine manufacturer's view, should deliver maximum efficiency with minimal wear on the engine's internal parts under the given “typical” operating conditions.

The most important of the oil's parameters is considered to be its viscosity.

In plain terms a driver can understand: oil viscosity is its ability to stay on the surface of the engine's internal parts while still remaining fluid. Sounds simple enough? But it's precisely viscosity that changes the most with temperature, being a “variable” quantity.

That's exactly why the Society of Automotive Engineers (SAE) developed a classification for engine oil by viscosity, which describes a given oil's viscosity at various operating temperatures. Essentially, this classification gives you the temperature range in which the engine will run safely, provided the engine manufacturer has approved an engine oil with those parameters for use in that engine.</content>