I found the manual here http://126.96.36.199/~uajaom/images/File/ ... 315-02.pdf
and you're right! Service schedule has the oil separator at 8000 hours as the longest interval period. The impellers are not even mentioned, except in the parts list. I sailed with one of these things, it ran forever, almost always unloaded, as you say, just humming away. We just followed the manual with filters etc, the big blue box was quite unremarkable and we had other fires to fight.
I suspect it's along the lines of the Rootes blower fitted to Screaming Jimmies, where the impellers don't actually touch anything and the roller bearings are oil lubricated. Relies entirely on mass flow to deliver the air.
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