I wouldn't use castor oil. Castor oil doesn't combust - at least, it doesn't in my model aircraft engines, I have to wipe the crap off the wings of my planes.
You have to use a low-ash formula oil. The highest mineral grade is JASO-FC and the best that I've found is Castrol Activ 2T - Valvoline makes one too but it's not as effective.
You can go for a JASO-FD rated oil but being fully synthetic it won't be the same and it'll be a LOT more expensive.
The oil chemically combines with diesel (diesel being an oil anyway) and modifies its combustion properties. Unleaded does similarly (it's more an emulsion than a chemical combination) - but in the other direction. Oil burns slowly so deepening the oil mix would lengthen the burn (more torque at lower rpm). Adding unleaded would decrease the burn time adding horsepower but not increasing the torque. LP gas would be similar (because it also burns fast).
Why use the low ash formula? Our DPFs, mostly, but also to ensure that the oil does burn pretty much completely. As the piston descends in the power stroke, the fuel is supposed to burn to completion before the piston starts to rise again. With a slow-burning fuel you wouldn't want to have it rise quickly - so a long, slow stroke is the answer. I haven't tried it - now that I think of it, I will - I don't think the engine will like high rpm with a bootfull of 2-stroke.
I'll put 600ml in my tank (150L) when I next refuel - probably during this week, got about 30L left in the tank) - and I'll take it out to the Charlestown Bypass and give it a hammering.