Hello Gil
I have run the OS.40FP with Fuel with synthetic oil only,I was using Cool Power 15% nitro fuel and the engine ran well of course I never ran it lean. I noticed that the engines crankshaft would corrode and I started to add castor oil to the mix and the problem was vastly reduced.I was doing mods on the engine to get more power and Rpm so I took it apart a lot to see how it was wearing. I got it up to 17,500 Rpm on a 9x6 apc. Now days I will not run any of my engines with synthetic only. Castor oil will protect an engine on a lean run because it will not burn like synthetic,the varnish it leaves is protection against metal to metal contact.
Juan