Originally Posted By: Olas
Putting a micrometer on my cam lobes, and putting an internal vernier down my cylinder bores both back up my thinking
How does that show a 10w-XX of any sort is actually optimal, and in comparison to what? I tell you right now that if you had a 10w-XX anything in a vehicle outside here right now, unless everything was in peak condition and you had a lot of luck on your side (and a working block heater), you would be leaving it sit there for the next week or until the tow truck showed up, whichever comes first.
This isn't a thick versus thin thing at all, but I rarely use a 10w-XX anything. The F-150 has it, but I don't need to do any measurements to determine there's no wear. It hasn't moved in ages.
If one wants a 40 grade, there are better options than 10w-40 in conventional or synthetic. There are better options than 10w-30, too. 10w-XX has jumped the shark, and that's beginning to be apparent in diesels, too, with 5w-30 and 0w-30 HDEOs.