I use straight 80 all year 'round in my O-200, and it is very happy. It was very unhappy with either of two multi grades. My O-360 seems happy with Aeroshell 15-50, which is what it ran from overhaul, when I bought it, so I leave it for now. But It'd switch to straight grade were there the slightest problem.
Both engines/aircraft have oil pan preheaters and engine blankets.