Bought a 98 Johnson 9.9 that looks like new, barely used. After running a little rough at first, now runs great, but leaking a greyish sludge from prop. Mechanic did pressure test-fine, compression test-good, and cleaned out carburetor but leak continues. Also checked gear lube which was a little dark but found no water. Leak is a slow drip that smells like gasoline. Any engine pros out there have any ideas?