Full castor is OK. It will leave your engine a brown mess on the outside, but it is a good lubercant for the engine. Some nitro would help though, especailly at idle. The engine may be a bit harder to tune without it, but there are a lot of guys that do run FAI fule in their four strokes. I would reduce the castor to18%. The four stroke doesn't have the raw fuel going though the bearings and such but is lubercated by blow by from the power stroke. Most of what is in the crank case and elswhere is just the lube. The castor will portect the engine in the event of a lean run.

If you have access to Model Airplane News magazine, the October 2009 issue has a great article on oil in glow fuel. I can't locate the article on line though.

Don