As Scott just mentioned, the oil lubricates the engine and ALSO helps to cool it. The oil does not burn during combustion, so you will alwsays have some oil residue to clean up. That's why electric flyers call glow flyers "slimers".

You want your engine to run about 400 rpm below peak, because it will lean out a little as the prop unloads in the air, and as you burn off fuel. If you start out at peak rpm the engine will run too lean in the air, pverheat, and you will damage it.
Dennis-