I would suggest you consider the three following castors as reliable and suitable for model engine use.

1. Klotz Benol racing castor, a higher film strength castor from http://www.klotzlube.com

2. Sig Manufacturing castor, a good AA castor at http://www.sigmfg.com

3. Overseas from the U.S., Castrol M castor

You will find all of these very suitable for your engine and much better than medicinal castor, which may have been processed a bit more than for good lubrication, and may have additives for taste and pallatibility. I agree with downunder that although we have some very good synthetics now, some high viscosity castor with a flash point of some 515 degrees is essentially unmatched for even today's fuels. I know of no single synthetic in routine use today in "modern" fuels that exceeds the capabilities provided by a good quality castor oil, period. Maybe there is a good reason that old Baker Brothers castor oil plant was built in New Jersey back sometime around 1886. :-)