Originally Posted By: JBinKC
Over the years I have seen quite a few past shortish used oil analysis on ST conventional shearing a grade to the high 20s using a 10w30 similar to a level that is usually consistent with a with a name brand 5w30 and have seen a couple 5w30s shearing below 8 cST. The question is how quickly does it get out of grade. With respect to wearability it still met the expected standard which is really most consumers goal.
That's not operating at a 5w30 instead of a 10w30, though. The oil meets its SAE J300 requirements when new, and has certain viscometric properties in service, too. At "normal" temperatures, i.e. at 40 C, 100 C, and most temperatures that aren't stretching the the cold cranking properties of the oil, a 5w30 and a 10w30 are the same and have the same KV and HTHS requirements under SAE J300.
The mention of "5w30" makes the comments confusing and misleading, since I doubt the cold cranking properties of the oil improved with use. If an oil sheared to a 20, fine. Additionally, there are 5w30 oils that are significantly thicker at operating temperatures than an ordinary ILSAC 10w30, too. The range of allowable viscosities is wide enough, and such products do exist on the market.