My Saito FG-14 produced quite a bit of "black goo" for the first couple gallons of fuel. It is also running 20:1 Amsoil pro. After a good breakin period the goo has all but disappeared. Now I just have a couple of tiny specks on the plane after an hour or so of flight time. The exhaust is simply a Saito 90 elbow out of the head, pointing straight back. No muffler, and it sounds SWEET, not too loud either.
Before someone beats me up: The Evo manual says NO Amsoil, so I am running it with Castrol MC oil.