Viscosity of motor oil (BMW 5 Series E34)

The viscosity of motor oil changes with temperature. As the oil warms up, it liquefies. This affects the adhesion and pressure resistance of the oil film. When cooling, the viscosity of the oil increases, and the internal friction resistance increases. This property requires the use of such an oil with such a viscosity that would change as little as possible with a change in temperature.

When the engine is cold, the oil must be thin enough to not impede the operation of the starter and to be able to penetrate all lubrication points as quickly as possible.

Viscosity is identical to the internal friction of the oil and is classified according to the SAE system (Society of Automotive Engineers), for example SAE 30, SAE 10, etc. Large SAE indices are assigned to thick oils, low ones to thin oils. But viscosity is not a characteristic of the lubricating properties of oil.