SF6 is recovered rather than vented because it is both costly and a heavily regulated greenhouse gas. The entire point of recovery is that the charge goes back into service, which means it has to come out of the machine in a condition fit to re-use.

An oil-lubricated compressor puts oil mist into the gas stream. Once that happens the recovered charge needs processing before it can be returned to a switchgear enclosure, and in practice that often means it is not returned at all. The recovery equipment has then defeated its own purpose.

Oil-free construction is therefore not a premium feature in this application; it is the minimum condition for the machine to do the job at all. Up to four oil-free single-acting cylinders on an S-type crosshead crankcase, with gas-tight construction and special sealing, is what that looks like mechanically.