Viscosity is one of the most important properties of engine oil. The viscosity of an oil is always marked on its barrel. Viscosity is measure of a fluid's resistance to flow. It is determined by the internal friction which resists the flow of adjacent particles in the fluid. As long ago as 1911, viscosity provided the basis for the first engine oil classification system and was defined in the Society of Automotive Engineers (SAE) classification system. Most of the oils used today are multi-grade oils. SAE 5W30 is an example of a viscosity designation of a multi-grade oil.
Measures of viscosity are based on two variables:
Dynamic Viscosity
This describes the engine oil's resistance to flow at low temperatures. Oils are divided into the winter viscosity classes 0W, 5W, 10W, 15W, 20W, 25W. The smaller the number in front of the W, the lower the viscosity of the oil at cold temperatures. Dynamic viscosity affects the starter speed when the engine is cold, for example. The lower the cold viscosity index, the easier the cold engine will turn over on starting.
Kinematic Viscosity
Kinematic viscosity describes the ratio between dynamic viscosity and the thickness of the engine oil at a certain temperature. SAE summer viscosity classes are classified at a test temperature of 100°C. Typical viscosity classes are 20, 30, 40, 50 and 60. The larger the number in front of the W, the higher the viscosity of the oil at 100°C.