Just to add to the compressor oil confusion, here's a bit of information that will make most breathing air compressor owners flinch. I had a gasoline engine driven 4 stage, 15 cfm, 4500 psi Davey on a trailer for many years, and I used synthetic compressor oils for all seasons except winter. The single grade synthetic compressor oils (such as Summit SL-500 or Chemlube 500) became too thick on cold days to be sucked up by the not so well designed oil pump system . Took way too long to get any oil pressure, and it concerned me about damage to the unit. Never could find a suitable multi-grade synthetic compressor oil, so I ended up using Mobil-1 synthetic motor oil in the winter, and it worked out great. The flash point and ignition point were within specs I needed for the compressor, and there was really no question about the lubrication quality. We were even blending O2 at the intake to produce up to 36% Nitrox.
There is no oil, synthetic or otherwise, suitable for use in your lungs. That's why we use filtration. I had good filtration, three tall towers, and the air tested to Oxygen compatible quality. I always sampled the air at maximum hours I allowed on the filters, just before changing the cartridges, so I knew the O2 compatibility was not just a fluke when filters were new.
I am definitely not advocating giving up good synthetic compressor oil for motor oil! I do recommend using the best synthetic compressor oil of the viscosity called for by the manufacturer. Unless you have a RIX.
"I get plenty of excercise just pushing my luck!"