FWIW, 20-weight oils have been popular in the US for two decades now, and we have some extremely warm areas, too. Millions of cars and millions of miles later, they seem to work fine—so much so that now there are 16, 12, and 8-weight oils in some cars. Heck, 5w20 came out in the mid-50s, and Mobil 1 pushed it in the 70s—but I guess engines didn't last all that long then, for the most part. The 1931 Ford Model A was recommended straight 20 if I remember correctly.
I know, for instance, of two famous Toyotas (one 2.7 I4 with 880k miles and a 5.7 V8 with 1M+ miles) that ran their life on 20-weight oil.
I understand some people fall under severe service (racetrack, always heavy towing, etc.), but it seems to work just fine for the average Joe.