I use synthetic oil in all my compressors. http://www.mobil.com/USA-English/Lubes/PDS/GLXXENINDMOMobil_Rarus_400.asp All of my compressors are outside of buildings and we have weather ranging from single digit to 110 degrees and a lot of periods of high humidity. Synthetic is a lot more tolerant to moisture than mineral oil. I have 3 or 4 Champions which I consider a premium compressor. I change the oil in all my compressors twice a year. I used to consider myself lucky to get 5 years out of a compressor. I switched to synthetic about 15 years ago and have not had a compressor to fail since. Mobil can be purchased from WW Grainger.