Scott, I looked at all the references on the net I could find and out of all the actual WWII pics I could find, color or B&W I notcied a few things...
Under the front cowl there was slight amount of oil/some fluid blown back toward the rear. Behind the engine cowl where the secondary cowl starts (the front of the kit without the cowl placed over the engine) looked a little greasy, maybe some soot as well on the lower half where the exhaust blew back.
Various
light streaking along the wing from leading edge, tapering back to nothing before it reached the trailing edge. I am assuming condensation dissolving dirt or such on the wing and blowing it back in flight. It was noticably around the oval access panels about mid wing across... one edge of the panel maybe wasnt flush as the rest and would cause the streak. not too many, only saw maybe 2 or 3 streaks on any carrier based craft.
NO exhaust streak along the fuselage from the exhaust... I noticed this on the newer SBD shots from airshows and what-not so started digging for wartime shots and nope... no exhaust streak... if anything the most I saw was some soot right at the exhaust pipe itself... kind of a roundish blob maybe got there during startup. actually in most of the shots the paint was LIGHTER where most builds show exhaust.. can't explain it unless it was due to washing when back on the deck...
I would be careful about weathering TOO much... even depicted during combat, carrier based planes were WELL maintained by the USN and USMC. They were very picky about even dirt on the exterior (I remember on my cruises how they would be up till late cleaning and maintaining the helicopters while deployed) Island based aircraft might fare a little worse as less support staff lacked and especially during battles...
I'll try to get a few resource links to give you an idea what I was talking about..