A bit of enlightenment for the multitude:
In order to help keep the lube oil from escaping the bearing sumps, a bit of air is "borrowed" from an early stage of the compressor and applied to the outside of a sump seal. Some air leaks past the seal into the sump, and in the process prevents oil from escaping.
But this air must now be vented - often through a centrifugal air-oil separator. This may be built right into the engine main shaft, and thus it's most practical to route the air out through the pipe that rosenblad noticed.
