Originally Posted By: tig1
How do you determine HTHS? Use the wt oil the manufacture recommends.
As I said, it's fairly simple to make a reasonable estimate of what the manufacturer has in mind, based upon their recommendations. When Ford is recommending a 5w-20, you know it's an ILSAC 5w-20, and not a Red Line 5w-20, which will be significantly different based upon HTHS. When GM calls for a 5w-30 dexos1, you know that a 5w-30 A3/B4 will be significantly thicker at operating temperatures.
Look at the trouble VW/Audi service departments have gotten into over the years by not being able to understand the difference between a 0w-30 or 5w-30 ILSAC versus a 0w-30 or 5w-30 A3/B4.
FordCapriDriver: There probably are some racing oils with higher HTHS that you could track down, but that Red Line 20w-50's HTHS of 6.1 is pretty high.
79sunrunner: Depending upon application, one has to be careful, particularly when looking at some older engines. An engine that specified a 10w-30 back in the 1970s wasn't necessarily expecting one with an HTHS of 3.1. It may not be a problem. However, a lot of engines back in those days specified a range of viscosities, so it's worth paying attention to that.
It's much easier to estimate what the OEM had in mind nowadays, when they specify one grade, and even with them having a chart from other parts of the world, with SAE J300 having been tightened up on the HTHS front. For the MG, if it were me, I'd probably consider a 10w-30 HDEO, rather than a 10w-30 PCMO.