How Is Viscosity Measured?
The viscosity of oil changes with temperature, therefore multigrade oils were developed to provide protection across a range of operating temperatures. The SAE (Society of Automotive Engineers) scale shows the viscosity of oil at both hot and cold temperatures. That’s why the viscosity grade on the oil bottle is made up of two numbers.
The first number followed by the letter W describes the viscosity of oil at low temperatures (the W stands for winter). The lower the number the thinner the oil. A thinner oil at low temperatures is good because it flows more easily and is therefore able to protect the engine when it is first started from cold. If oil is too thick when cold, it will not circulate freely and will reduce fuel economy.
The second number describes how thick the oil is at the engine’s normal operating temperature. The higher the second number, the thicker the oil. If it's too thin when hot, it may not protect effectively. If it's too thick, you lose efficiency.
The correct viscosity grade will be displayed in your car handbook.