Percolation? Heat soak? Don't drive during summer??

Only thing I'll say about 20W-50 (because this is a cooling thread, not an oil thread) is that in the '80's (is that "back in the day"?) my 383 A-body highway fuel consumption improved from 14.3 to 14.7 mpg, just from switching to 10W-40...
So it does take more power, which means more heat wasted in the coolant.