I don't see any problems with using a little castor oil in the fuel. Castor oil does wonders for protecting the ferrous metal parts from corrosion where synthetic oil does nothing. So usually I can run a 4 stroke engine for ages before I have to replace bearings. But I see other guys all the time replacing bearings every year and they use 100% synthetic oil.
Now granted if you let the engine sit for a long time the valves tend to get sticky and you might have to work them loose again. I also don't see using 100% castor oil as it tends to gum up the valves faster. Anyway I use a blend of synthetic and castor oil in my fuel.