Man oh man, the average diver seems to be super leery of everything and anything. You would think that compressors are pumping out asbestos or (horrors) nicotine. The main 'contaminent' that must be separated out is water. This is followed by the small amount of residual oil vapors. Oh yes, don't forget carbon monoxide from gas engines. This must be avoided and can't be filtered out despite claims about chemical catalysts like hopcalite.
I wouldn't buy one of those Italian made MaxAirs that use chicken wire for the fan guard. Some of those ex military compressors are cheaply priced and very nice indeed. Avoid the one with the 1 1/2 hp military engine. You need 5 horse power, either Honda or new MIL SPEC. These compressors come equipped with condensators. All you need is their ridiculously inexpensive after filter canister which uses a Laurence Factor cartridge. These are packed with multisorb and will knock out any oil mist/odor. The resulting grade E air is cleaner than anything found in one of our cities. You guys breath that stuff 24/7, right?
Before you order your compressor check if it has a 'snorkel' type intake. If not, just ask for one, they have. You attach a six foot, 1" clear plastic hose with a paper element air filter which is hose clamped to the end. This filter is available from Lausen products and is normally used as a small engine intake filter.
Make sure you drain the condensator (small tank on the compressor) every 1/2 hour or every fill. To extend the life of the valves and condensator, run the compressor under NO load (drain open) for 30 secs after the last tank is filled and before shutting down.
One more thing, these military compressors do not come with a back pressure valve. This simple device is absolutely necessary to ensure that all water is wrung out by the condensator. Inquire. Install the valve on the condensator. Have the valve set to 1200-1800 psi.
Most military equipment of this type use #4 JIC flared connectors. It would be best if your fill hose, which will probably be ordered separately, has these connectors, at least on the compressor side. The Stewart Warner backpressure valve, a common military type will have this type of fitting. For some reason, the honchos that sell surplus compressors seem to like 1/4 inch pipe thread on their hoses. This setup is inconvenient for a portable. The flared hose ends can be instantly disconnected countless times and still form a reliable seal. The only place you need a 1/4 inch pipe fitting is on the condensator/where the back pressure valve screws in. This is a permanent install, no removal necessary. Even here, the later condensators often avoid pipe thread and use a JIC compatible thread (3/8-24 O ring).
This sounds complicated but it is not. Some running around will be necessary to locate the few extra parts. The overall cost is less than half that of a Maxair. Parts and service are available for most surplus compressors, especially the Stewart Warner types.
Anyway, don't obsess about air purity; follow this advice if you decide to get a used unit for less than half price but be prepared for few hassles at the outset. Use common sense in operating your compressor regardless of origin and history.