That is the latest conventional wisdom at work then.
My Lexus V-8 requires 10-30w. I put in 10-30 wt.
You know why? Because that is what they recommend *and* I have worked on those engines enough to know that a camshaft bearing clearance of .0008" damn well needs thinner oil.
The VW engineers suggested 30w in the summer and 20w in the winter for the 1970 bus. They warn against high speed driving with 10w oil if ambient temps climb above 32*F. That is pretty much it.
The later owners manuals do discuss multi-viscosity. There's 20-50! Damn, they recommend straight 40 for the highest ambient temperatures. They warn, again, not to do sustained high speed driving if the indicated viscosities are not sufficient. Apparently, anything above 50* ambient is too high for your chosen oil viscosity. Take a look:
I have decades of hot highway driving and can hear my main bearing clearances open up (waaaay past anything the Lexus will ever see). If the "vast" majority you speak of knows something I don't about the critical oil wedge that must be maintained inside of a rapidly spinning bearing journal, by all means recommend that they get in touch with me to school my stupid ass.
Meanwhile, it is not all about oil temperature reading dick-sizing amongst the vast majority who do not know what they actually *need* for a happy hot long-lasting engine. See how it fills me with grave doubts to be intimidated by the vast majority?
Colin