Well, this may be 'just superstition', to be sure, but I tell people to change compressor oil every three to six months, depending on the level of service the compressor is used for.
I say 'superstition' as I am woefully ignorant of oil chemistry, and really have a very limited understanding of the ways in which oil may degrade over time, heating/cooling cycles, etc.
Given such ignorance, I think it easiest to think of oil changes as 'insurance', on the theory that oil is much cheaper than bearings, and the value of the working time needed to pull a pump down for overhaul.
This much I can say, from first-hand observation. I've pulled down some number of the old recips which had the heads carboned-up nearly solid, presumably from using the wrong oil, and seldom if ever changing oil. Some of these were marginally cost-effective to rebuild, given the amount of working time needed to literally drill and chisel out the hard carbon formation in the discharge porting.
(I particularly remember one 10hp Quincy which had been used in an automotive shop for several decades, with the suction side piped outside the building, but without a filter. It had bad ring/bore wear from....presumably....fine 'road dust', which would be ultra-fine silica particulates, a rather nasty abrasive, so, eventually, a lot of oil got past its rings. It had, apparently, been run on some sort of detergent automotive engine oil, and still ran smoothly, but didn't deliver very much air....the head/valve porting having become nearly a solid block of hard carbon.)
Allegedly, the stuff sold by the major oil companies as 'compressor oil' contains some additive to reduce such tendency to carbon formation.....I say 'allegedly' as I've no knowledge of the oil chemistry involved.....but, if it works.....well, its 'cheap at the price' if it keeps the compressors doing their work for a good term of service.
cheers
Carla
