ORIGINAL: downunder

Engines don't have sweat glands so wind chill isn't a factor in their cooling.

You could break in an engine in the air but only if you were absolutely certain it was holding the correct mixture the entire flight. That's much easier to do on the bench especially as the fuel level in the tank goes down. I might add this really only applies to an ABx type engine where the best initial mixture is around where the engine is just breaking from a 4 stroke into a 2 stroke so there's enough power to get off the ground. With a ringed engine in a slobbering 4 stroke (initially) you could end up doing a fast taxi around the field .