I used to use 50w Castrol "R" in two stroke Yamaha and Bultaco 250cc short track (flat track) motorcycles. When you used Castrol R mixed with gas, it got stale if mixed over a few(I don't remember exactly how many) hours and any time ambient temperature went below 60 degrees F, it wanted to separate out. The colder it got, the quicker it would separate out of solution. It was rumored that once you switched a motor to castor oil you couldn't switch back without engine damage(I never experienced this, but maybe I just got lucky).
However, 1)in a mix it would consistently make 2-3 more horses on the dyno than the available alternatives in the late 1960's. (38hp/with, 35hp/without), 2) It would tolerate more heat before breakdown meaning that if you leaned out the fuel too much and melted a piston crown it usually wouldn't gall the bore like dino based oils often would, and 3) motors its been used in stay oily and rust free for years(unlike popular synthetics at that time that would allow an unused 2 stroke motor to rust internally in just a couple of months. )
The speedway bikes you refer to were 4 stroke motors that used a total loss oiling system that pumped the oil through the motor and out on the track. Kind of messy, but they threw so much dirt
or pea gravel that you never noticed the oil.
I believe that castor oil would have to be drained out of a motor after each days use if it were contained in an oil pan or sump, but this is a guess. I suspect it might tend to gum up the rings if a motor was not pulled pretty hard. It was a terrific racing oil years ago, but its quirks made it less popular than it probably deserved to be.
When used in 2 stroke motors, it burns with a very distinctive smell.
Illflem, when its used in motorcycles, it must not produce the ricin
gas or there would be a lot fewer old short trackers around.