Tom C,
As far as I know rust is the end result of corrosion process. Therefore it doesn't matter if you use cartor/sinthetic/blend. In fact nitrometane is the responsible for rust. It is very aggressive to iron and steel.

Tailwheel,
In general I use 10% nitro and 18% synthetic, I left behind castor. I had a couple of installation where I couldn't use afterrun and that is bad. In general, since I stop using home made fuel (no nitro and long time ago....) all my engines present some rust in the bearings and crankshat.