Oil mist?
I believe the pnuematic system was driven by suction so that the "wet" pumps did not blow oil mist into the instruments. When the dry pumps came along, some aircraft came with a pressure system, but I do not think that lasted very long.
Instrument LAME's used to say cigarette smoke was harmful to gyros.
It would probably not be a good idea to fit a wet pump on an aircraft with a pressure system to drive the gyros.