Oil vapor... The accumulated oil in the lower cylinders are ejected into the exhaust manifold where it vaporizes.
I was hoping I wouldn't have to go into detail, but here it goes...
If a radial or inverted-V engine hasn't been rotated for a day or more you'd need to hand crank the engine a few revs to pump the oil out or you'd risk liquid lock. You can never be too careful with inverted cylinders. I'm sure you've seen ground crew hand pulling revs on the props of B-17s and other radial engined planes plenty of times in videos, or even in the movies.
The oil comes out of the big Pratts and Wrights so slowly that you'll pull through a couple of blades fairly easily and then it suddenly comes to a stop, and then you're heaving, or rather leaning on the blade as it moves ever so slowly as the oil pours out, then frees up for another few blades until it comes to the oiled pot again which pulls through with less resistance than before and produces another run of oil, and the third time around the resistance is pretty even. Then you can try a start, but its going to be a while running on 8 or 16 before the fuel washes the plugs on the bottom cylinders clean enough to fire. Nine blades by hand in the normal direction of rotation on a DC3 to make sure it was OK to start. Even after pulling through 9 blades there's gallons of the stuff still swilling around. If you encounter a liquid lock remove lower plugs and move the prop until all the oil is out.
Also, with many radial engines, if it's not run for more than a few days it's cowlings off and take out the drain plugs in the inlet pipes of the lower cylinders. Oil can collect there and it won't come out by hand pulling it through as the "elbow" in those inlet pipes is below the level of the exhaust valves. So any oil stays there until start up, whereupon it gets sucked into the cylinder where it can cause a lock. If you are lucky, the engine breaks there and then and you get you wallet out. If you are unlucky you bend a con rod and the engine fails some time later, and somewhat dramatically, as the rod breaks in the air one day.
Oil consumption in flight was also higher with inverted cylinders, radials in particular, for the R3350 (B-29) acceptable oil consumption in flight was 3 gallons per hour. The B-17 has a 37 gallon oil tank per engine.
If you for some reason don't take my word for it:
http://www.youtube.com/watch?v=UD910YCkLn0