It's a curious situation: most Fox 35 engines have blackened stuff all over the cylinder head, while my OS35FP, OS40FP, and even LA46 all have at least a couple hours running, and the engine still looks like brand new!
What is it? I run SIG 10-10-10 in nearly all my engines, even the RO-Jett, which is supposed to run on all-synthetic oil.
Maybe the Foxes have tiny leaks all over to let castor oil escape?