This only has to do with oil sump pad heaters - hot oil, and cold upstairs
in the engine - when the water in the oil is evaporated it condenses upstairs
and you get rust. Not an issue if you heat the entire engine compartment,
or the entire engine (i.e. cylinder band heaters in addition to the sump)

Jeff

Quote Originally Posted by Esser View Post
The owner of the Cherokee i fly is an AME (A&P for you southerners) and he says only to preheat for a few hours before you go flying and that preheating all the time isn't good for the engine either. Why this is I don't know but I will ask him next time I see him