Before the 1960s, all oils were monograde. Multigrade oils were developed have a designation in the format 20w50. Mulitgrade oil was made possible by the use of polymers, that expand as they get hotter. Therefore a multigrade oil can have the viscosity of an SAE20 when it is cold and the viscosity of an SAE50 when hot. This removes the necessity to change from thin oil in the winter to thicker oil in the summer, and maintains the best protection for the engine over a wide range of running temperatures. The 20w number is the cold viscosity and the 50 number is the viscosity at 100°C.
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