Why are motor oils referred to by weight? Every so often I am asked this question when explaining SAE viscosity grades, and the answer is a bit of a history lesson.
In 1911 the Society of Automotive Engineers (SAE) adopted the first classification for crankcase oils called Specification No. 26, where oil was specified by specific gravity, flash and fire tests. It was the first iteration of what eventually became the J300 standard and included specifications (not grades) from 20 to 115, commonly referred to as weights.
In 1923 a specification for 10 oil was added, and all oils were classified on the basis of their viscosity ranges measured by Saybolt viscosity at 100 F and 210 F. With this specification, for the 20 to 50 oils, their number represented the first two figures of the average Saybolt viscosity at 100 F. For example, the lowest range was 180 to 220 SUS, and the average of those values is 200 SUS; therefore, this oil was designated as a 20, the first two digits of 200. The numbers from 60 to 115 still signified the average Saybolt viscosity range, but at 210 F.
1923年，补充了10号机油的规格，所有机油都根据其在100℉和210℉下用赛氏粘度法测定的粘度范围进行分类。对于20至50号机油，根据该规格，数字指100℉下平均赛氏粘度的前两个数字。例如，最低范围为180至220 SUS，并且这些值的平均值为200 SUS；因此，这种机油被命名为20，即200的前两位。60至115的数字仍然表示赛氏粘度范围的平均值，但是是210°F下的平均值。