hi surely its the damp/water in the air that causes the corrosion, as far as i know any oil coated surface stops corrosion- never seen a rusty sump in a car engine, if this guys engines are going rusty they must be shelved for a hell of long time so he cant be much of a flyer can he?
if anybody is going to worry about an engine going rusty cos they not going to use it then they might as well sell it, engines are fairly priced, just not worth worrying about, sounds bit of a tight-ass to me, just my opinion