I've always used fuels with all or high percentage castor, never used afterrun oil, never had internal rust, even when the engine hasn't been used for years. If they gum up setting, a few minutes in a 250 F oven frees them right up. I've only changed 2 sets of ball bearings because I neglected to clean them adequately after vertical landings in clay.

In fact, I once had a Fox BB .15 RC sit for almost a month in a puddle of water that it shared with oak leaves. Oak leaves are very acidic. When I found the remains of the plane, the engine and muffler were full of brownish water. Many metal fittings on the plane, including some of the servo wiring, were scrap.. I drained the engine, flushed with clean water and fuel, and couldn't find a bit of rust or corrosion outside or inside, not even in the balls. Nothing protects like castor.

In fact, I have some engines from the 1950s that were run on Fox Blast and Hi Nitro (IIRC, around 40% nitro) that were put away wet that are still not showing corrosion.