Air compressors that are portable frequently use 20 or 30 weight non-detergent motor oils. The 20 or 30 weight non-detergent motor oil can be found at most regular auto stores. It is not necessary to purchase oil labeled as compressor oil, and some manufacturers of air compressors even approve the use of 10W-30 synthetic oil in their machines.
Prior to purchasing an air compressor, be sure to keep in mind what the primary use of the machine is going to be. If the air compressor is to be used on a job site, think of whether an electrical outlet is readily available. If one is not, a portable air compressor that uses oil is a good idea.