This i a reply i "stole" from a former post on this forum, so thanks to who ever posted :-)
"In July 1926 the Society Of Automotive Engineers (S.A.E.) crankcase oil viscosity numbers were adopted. (ie. SAE 30)
By late 1933 Winter oils were classified in accordance to their viscosities @ winter temperatures, these are marked without the SAE but with a 'W' . (ei. 20-W ) Multi -Grades were available by 1957 ( ei . SAE 10 W 30)
The second system of motor oil classifications was established for the petroleum industry by The American Petroleum Institute (API ).This system classifies oil by types according to the design and operating conditions of the engines in which the oil is to be used.The first types were REGULAR,PREMIUM and HEAVY DUTY.In 1952 API replaced these types with:
ML - light duty
MM - medium duty
MS - severe duty
DG - severe (diesel)
DS - extremely severe (diesel)
DM - severe(diesel) was added by 1957 ( DG now becomes NOT severe)
Some oils are designed for both gasoline and diesel service and may be identified "FOR SERVICE MS - DG (1958)
