As David also wrote and [link=http://www.osengines.com/faq/product-faq.html#q2]OS says specifically[/link], one gotta be outa his mind to use all synthetic, American made fuels (except Cooper's Ester based oil) in an AB(X) engine...

This has repeatedly been proven to be a problem.

I you don't overheat your engine, you will not get any of that brown stuff on it either, even with all-castor.

It is your engine's life insurance, as well as its livelihood.

If your engine is cowled-in, remove the cowl, or add internal baffles that will force air to flow over the engine.
Otherwise the air will take the easy way out, without caring that it is supposed to cool the engine...