All oil has a "flash point",the temparature it'll burst into flames when heated..just under that temp is when oils will break down and carbonize ..thicker oils and synthetics have higher flash points,and that is what you'd want in an old engine that burns oil--the higher the flash point the less oil will be consumed..
I'm surprised air cooled engines dont just BOIL the oil,I would think they should have added cooling fins to the crankcase,but evidently aluminum dissaptes the heat enough so it wont..
Some engines do duct some cooling air under the crankcase for additional cooling. After cutting the lawn with my 16G (when it was still growing b4 the drought), I checked the CC temperature when I was done, it was 185 degrees on an 85 degree day. That is with a fairly light load, would probably be higher in the spring when the grass is thicker. This is on a Kohler Magnum 16 twin, I should add.