In fact the Castos oil as lubricant is cooling much better than any synthetic one, and also delays the ignition timing.
Of course if you keep the 30% nitro fuel as a must, then the simplest is to place a second washer inder the head,
as they do in the helli-engine versions. Still with 30% nitro the engines get "older" much faster especially if they
are not cooled very well inside; that is what the Castor oil can only do. I have no experience with 4-stroke engines
and if there is something specific with their fuel; I gues only that high nitro % is getting you only more stable low
rpm but not much more power (if any at all) if you do not prevent the detonation due to premature ignition.