Generally speaking, the only engines that require castor oil are the plain bearing engines (bronze bushings instead of ball bearings supporting the crankshaft) like the O.S. Max La-series, the K&B Sportster engines, or the Thunder Tiger GP-series engines.

Castor oil's thick viscosity helps float the crank shaft inside the case and helps seals any minor gaps or spaces. Castor oil offers ideal lubrication and protection for almost all kinds of glow engines (YS engines being the exception), but most pilots prefer synthetic oils in their glow fuel because the exhaust residue is lighter and easier to clean up.

I run 10% nitro fuel with 80/20 synthetic/castor lubrication through my Saito 4-strokes just like I do with the rest of my glow engines. It's still running like a champ after half a decade of regular flying with castor oil in the fuel mix.