If these engines have been run a lot on all Castor fuel trying to run them on a synthetic Castor mix will soften the Castor varnish and cause them to seize. Ask me how I know! I run all my newer engines, Brodak, OS, Double Stars on Omega 10 containing 8 oz of Castor. Morgan measures their oil content by weight, and according to my calculations and an actual experiment that Randy Smith told me about this should give a total oil content of 20-22% oil, half Castor and half synthetic.