I do fully understand the difference between “winter” and hot ratings. Of course, they’re different, and of course, for proper protection across the range have to be adequate.
Yes, more film thickness, all things being equal, should provide more ultimate ability to keep metal surfaces separated. But if the engine involved does not present conditions in which the added protection will ever make a significant difference, then how does it really matter? If there’s fully sufficient MOFT across the operating range using a lower vis oil, then why use a thicker one?
This is hard to believe. I joined this site TWENTY years ago, and we’re still chasing our tails, freaking out, completely losing sight of when the differences between viscosities matter, and when they may not. In the case of the aforementioned Coyote, Ford looked at the results, and wisely admitted that there WAS a practical reason to shift back to the 5w-30 oil. Against that specific good example of when there is a difference, we now have ~25 years of experience with legions of vehicles which have lived long, unremarkable lives using “thinner” oils their whole lives. There’s a “metric” that nobody can ignore.