I agree that congealed oil can sometimes seem like rust, and give a jerky feel when rotating the engine. It's pretty hard to tell the difference with out opening the engine.

Another side effect of running an engine that has a little rust inside is that the rust acts as a lapping agent and really wears down the piston and liner fit, causing a loss of power. This is espeically true in ABC engines. Ringed engines are more tolarant of that sort of thing.