Quin-Cip is a mineral oil specially formulated for air compressors. I've used this stuff in marine duty air compressors that see hard and heavy use with nothing in the way of piston scuffing or ring wear and little to no ash deposits on the valves. I'm not saying to buy from these folks as the price seems really high, but this should give you an idea: http://www.tptools.com/p/3434,89_Quincy-QUIN-CIP%C2%AE-Compressor-Oil.html
About a year and a half ago, my boss squealed about the cost of a pail of this and had us start using Mobil DTE 26 hydraulic oil, and the top ends of the compressors are noticably more coked up that they were on the Quin-Cip.
Tom