that would be my guess (fuel+ethanol) based also on previous issues seen around OC, but i'm not affected to that kind of fuel cocktail so can't add much.
This one clearly (or darkerly) seem to be oil/soot deposits from missed combustion...
Wether is a ongoing problem, or remnants of something fixed before, it's unknown..
Growing old is mandatory... growing up is optional!
Raid de L'Amitiť 2011 RR (link