Frank,
This should help.
http://stunthanger.com/smf/index.php?topic=4037.0
Here is a excert from that article.
So what percentage do you try? For motors like Fox .35s, OS Max 35s or the old McCoy’s and K&B’s, use a fuel with 26 to 28% oil content; preferably half castor and half synthetic, up to 75% castor is OK. These motors have very small bearing surfaces, and are subject to much wear and heat, most are all plain bushing motors and most have unbushed rods. They need a lot of oil to help cool the engines. Since these motors run hot, they need extra oil to keep them lubed,clean, and to carry out heat . If you have one of these that is in very good shape but, is just starting to get some brown or black varnish plating on it, the synthetic mix will clean it up for you, resulting in increased life. Do not use the synthetic blend in an old motor that has a lot of time on it with all castor fuels; the synthetic will remove the castor varnish off the piston and sleeve and will in some cases, leave you with the worn-out motor that had to start with. Also always try to NOT use prop shaft extensions with these engine, as it adds a lot of wear on the crankshaft bearing.
Also approximately every 4oz. of castor will will raise the oil content
in a gallon of fuel 3%.