Multigrade oils were developed in the 1960’s and have many different formats (20W50, 20W60 etc.). Prior to this, all oils were monogrades (ie. 30, 40, 50).
This was made possible by the invention of polymers. Polymers are molecules 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 a thin oil in the winter to a thicker oil in the summer.
The 20w number is the cold viscosity and the 50 number is the viscosity at 100 centigrade.
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