MVVS .15 Diesels are renouned for "dirty" exhaust as they run in. If you're sure it's not overcompressed then don't worry too much, a friend of mine describes the dark exhaust as being due to them using "dirty" cast iron in the piston.

I agree if the fuel works as is in the ST G20-15D, leave it alone. Trouble with storing diesel fuel in metal containers is usually rust blooms in the tin plate. Can you normally smell Ether where you store your fuel? If not you don't have a problem.