The viscosity classification of a motor oil according to the system developed by the Society of Automotive Engineers and now in general use. "Winter" grades are defined by viscosity measurements at low temperatures and have "W" as a suffix, while "Summer" grades are defined by viscosity at 100ํํํํํํํ°ํ C and have no suffix. Multigrade oils meet both a winter and a summer definition and have designations such as SAE 10W-30, etc.