In the long run, I don't expect choosing one over the other will affect engine life at any perceptible level, even to run a high mileage dual test and to pull down the engines and mic them. The 5W or 10Ws will like the cold a little better than a 20W.
If I'm not mistaken, there are people around here running the 40w in big bores, likely with higher heat issues than stock, who get good oil analysis results that aren't showing thermal break down.
Just my opinion -- As long as we keep the oil reasonably clean, let's say with a 3 to 5,000 mile change interval, and use one of the four or five oils mentioned around this site, I believe "it's all good."
