An oil's viscosity measures its ability to flow. Viscosity decreases as temperature increases. The rate of this change is known as the viscosity index (VI). The higher the VI, the lower the rate of change of viscosity. Polymer-based additives have been developed to improve the VI of lube oils.
An engine lubricating oil needs to have a low enough viscosity to flow readily at low temperatures, and a high enough viscosity to provide suitable lubrication at normal working temperatures. Accordingly, multigrade oils have been developed. These act as a low viscosity oil at cold temperatures, and a high viscosity oil at higher temperatures. Basically, a SAE5W50 oil would act like a SAE5W oil at low temperatures, and like a SAE50 oil at higher temperatures, thus ensuring adequate lubrication throughout the operating range of temperatures.
