They used to recommend 20 in the winter and 30 in the summer. Then about
50 years ago they introduced multigrade oils so that you didn't have to
change between winter and summer. Unless your engine is pretty worn, my
guess is that 10W-40 should be more than sufficient and heavier oil would
only reduce the flow of lubricant to places that need it WHEN YOU START,
not 5-10 minutes later. Remember that unless it is extremely hot (110° F,
for example) your oil is expected to run about 75-100°F hotter than the water.
It is made to do that. The people who design the viscosity-leveling
additive packages and feedstocks know all about this and they work very
hard to make oils that are good all year and in just about all temps.

Now if your engine is running at 180°, then your oil is running at 255-280°
under long-term operation. How much difference does it make if the outside
temp is 80° or 100°? Not much in my book? The hot oil sees it as much cooler
and the exposure to the airstream cools the pan, which in turn cools the oil.