Originally Posted By: Shannow
Originally Posted By: DWC28
When SAE was adding HTHS viscosity there was much debate on what the value should be for 40 weight. The diesel OEMs wanted a higher value than the passenger car people. The debate was strong for both sides. The only way to settle the argument was to have a higher value for 15W-40 to please the diesel people and a lower value for 10W-40 to please the gasoline guys. Now we have two different groups of 40 weights. The ones that were originally meant for diesel engines one originally meant for gasoline engine. Thus diesel manufactures did not recommend 10W-40 or 5W-40 or 0W-40 and gas guys don't call for 15W-40 or 20W-40 or 25W-40. Look up SAE J-300.
The original inclusion of HTHS in J300 had 2.9 for the 0W/5W/10W-40s, and 3.7 for the 15Ws and up.
Apparently they had to drag the diesel engine manufacturers kicking and screaming in the day to go to multigrades, and the "natural" HTHS for an SAE 30 is around 3.5 anyway, so you can sort of infer where the mins came from...obviously a properly built 15W40 can seriously do better than that minimum.
There WERE M1 0W40 bottles down here labelled "energy conserving", a long long time ago, so presumably they were at the "30" end of the J300 40s back in the day.
Well, some truth to the "kicking and screaming" thing regarding multi vis oil use recommendations by some diesel OEM's. But keep in mind, there were a LOT of 2 stroke diesels around, even up thru the 1980's. Those did require a straight weight oil in them, and still do for the few that are still around.