Multigrade Motor Oil

The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. A specific oil will have high viscosity when cold and a low viscosity at the engine's operating temperature. The difference in viscosities for any singlegrade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oil. These additives make the oil a multigrade motor oil.
The idea is to cause the multigrade oil to have the viscosity of the base number when cold and the viscosity of second number when hot. The viscosity of a multigrade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.The API/SAE designation for multigrade oils includes two grade numbers for example, 10W30 designates a common multigrade oil.