The castor is very important.

You're completely out of luck at finding Fox Super Fuel. It was the fuel of choice back then because it had 25% oil, and every bit of that oil was castor. Truth is, it's not really life or death nowadays to have that high a percentage. Super fuel could kill those engines just as dead. It wasn't magic.

Look for a fuel that advertises some castor oil content. It will work. BUT

You really can NOT allow the engine to go lean, too lean, or you will hurt it.

And use either 5% or 10% nitro fuel. Don't go higher in nitro.

Those OS35S were made when most modelers were taught to pay attention to needle settings. Fuel without castor have no safety margin on the needle setting for those hard piston engines. You're going to have to pay attention to the needle.

Do it and that sucker will pull stumps for about a year of steady flying. I've kept the crankcase of all the OS35Ss that I used until the shaft bearing was history. They're in a cardboard box. It ain't a small box. grin.............