I sort of feel bad every time I post this because it is not anything like what the experts say.  I am running a Fox stunt 35 I bought back in 1977.  The only things I did to it were to lap the piston and cylinder and put in a Super Tigre NVA.  Back in the day, it had well over 1000 flights on it with (probably) all castor fuel.  I've been flying it for about three years now, well over 200 flights.  I am running Sig Champion 10% nitro, 20% oil-half castor, half synthetic.  During this period, I put one lean run on the engine.  It got hot enough to stop in flight.  No damage was done that I can see.  The engine has excellent compression and gives one flip starts.  I have no intention of changing fuels.