I agree that 8% is a safe minimum oil content but......

Most castor oils are thicker than most synthetic oils and castor does not burn compleetly with the combustion. Thats why I think with pure castor based fuels it is easy to go down with the oil content.

Regarding the difficulties in tuning. Because the thickness of the castor oil and it gives a layer of oil inside the engine I do think it will also act as a piston ring, a fill up between piston and sleeve to build up compression. When the engine is near end of life you can notice difficulties to get it reliable running when using some fuels and a perfect running engine with others, the other fuels must have a high content of castor oil.