FWIW, OS's manual that comes with the LA series engines recommends a 20% all castor oil fuel with 5-15% nitro. As a practical matter, the larger LA's run fine and produce decent power with 0% nitro although they do produce noticeably more power with 5%.

I have a LA .46 that was always run with an OS spec fuel I mix myself, and its still got a ferocious ABC "pinch" at TDC even after about 5 hours of running.

For his whole life, Duke Fox was a proponent of castor oil for his engines, and they'd last for many hundreds of hours when fed what he said to feed them.