I"m baffled by this. I thought Karts used more oil, not less. The old Yamaha 100s run at 16:1 or 20:1 (basically as much oil as you could get without fouling).
Anyway, keep in mind castor oil will work fine in your bike and will smell good, but even oils with only a fraction of castor may tend to gum things up (regardless of what the bottle says). This is not a problem if you do teardowns/maintenance on a regular basis, but I thought I'd mention it.
Also, see if it says anything about cold weather...some oils with castor tend to settle out in cold weather.