There could be some grey, whispy cordite streaks (not oil) from the gun muzzles, although the crew-chief would normally try to keep the leading edge of the wing, on a 'natutal metal' bird, as clean as possible. Any streaks would show more on a painted surface. There could also be some thin streaks from the shell case ejector slots on the underside of the wing. Have a look at some reference photos, but from what I've seen of the particular aircraft you are modelling, there was little in the way of weathering or dirt. The 78th Fighter Group only recieved Mustangs in January 1945, so they only saw service for four months, and most shots I've seen of their aircraft, colour and B&W, have shown fairly clean machines.