OK, does anyone remember the rating system BEFORE the API ratings?
"ML" (Motor Light) Gasoline engines, light duty service. This one was the traditional non-detergent stuff in the 1940's and early 50's. It was often sold from 1-quart glass bottles with screw-on tin funnels, transferred from a 55 gallon "bulk oil" drum.
"MM" (Motor Medium) Slightly heavier-duty oil, usually sold in sealed cans, still non-detergent, I believe.
"MS" (Motor Severe) Straight weight high detergent oils, and the earliest of the multi-viscosity oils like RPM "10-30 Special".
"DG" (Diesel General) General-service oil for small and medium-sized Diesel engines. Usually similar detergent qualities as MS, but with somewhat higher film strength for Diesel rod bearings under high compression loads.
"DS" (Diesel Severe) Highest quality hi-detergent oil available, intended for large Diesel engines under heavy-load highway and off-road use. RPM DELO (Diesel Engine Lubricating Oil) was one of the top oils in the mid to late 1950's. Usually a straight-weight oil, but 15W40 DELO became available in the late 60's.
Jerry
