There was no staining coming out of those vents despite what you see so many modelers do to the contrary (also, the people who did the "weathering" for the Memphis Belle movie did the same thing, making some people believe that is correct. It is not.). There was only warm air coming out. So many modelers have "exhaust" stains behind those. To start with, the exhaust exits through the turbos. You know, on the BOTTOM of the wing
... Most of the staining you see on the nacelles is from oil. For whatever reason, the crankcase is vented inside the cowling. Initially, the oil starts staining the cowl at roughly the 11 O'clock position as that is where the vent is located. As the airplane builds hours, the oil blows around inside the cowling and so you get more oil staining around the cowl. Plus radials just naturally leak oil, so more staining appears. You will occasionally see some exhaust staining on the cowls but that is generally brownish or blue tinted from overheating. You don't allow those to continue because otherwise the exhaust gases will cause serious damage. Back to the oil stains: You will see in photos from above, the oil flowing around and between the cooling air slots.
As for the contrails, they have two sources. The hot air coming from the exhaust and from the vortexes caused by the prop tips and the wingtips. The air coming out of the vents is not that warm.
Here is where the crankcase breather is located:
In this photo, you see nice new Forts waiting to be delivered to bomb groups after being ferried to England. And the oil vent has started making a mess to the left of center:
In this well known image of "Miss Donna Mae" going down over Berlin, you can see how the oil blowing back varies on each engine due to the prop wash and airflow off of the fuselage:
Here is an airplane with some hours on it and the darkest streak is still more or less lined up with the breather. No 1 has some other issues and 2 and 3 have dumped enough oil that it is staining behind the cooling slots but the slots themselves are clean:
