Years ago, most vehicles used one viscosity grade oil in the summer and a different viscosity grade oil in the winter. The measurement of viscosity at high and low temperatures are properties of multi-grade oils. Motor oil viscosity grades are based on a scale developed by the API (American Petroleum Institute) lubricant organization. The values are defined in a specification known as API 1509 and are based on the resistance the oil gives to flowing at two different temperatures – cold and high temperature.
Viscosity is the resistance to flow of a fluid.
