Viscosity refers to the degree of fluidity of engine oil. The fluidity of oil varies with temperature. As the temperature rises, it liquefies. This affects the durability and resistance of the lubricating film. When cold, it becomes viscous, while its fluidity decreases and the internal friction resistance increases.
When the engine is cold, the oil must be thin enough so as not to overload the starter and so that it can get to all lubrication points as quickly as possible when starting the engine. This requires the use of engine oil with a viscosity that changes as little as possible with temperature changes. Fluidity or viscosity is equivalent to the internal friction of the oil and is designated by the SAE system (Society of Automobile Engineers), for example, SAE 30, SAE 10. Large SAE numbers are thick oils, low numbers are liquid oils. But viscosity does not at all characterize the lubricating properties of the oil.