Like davbell22602 said, any compressor oil will work... But if you can find it use synthetic like this at TSC...
Ingersoll Rand All Season Select Oil, 1 Liter - Tractor Supply Co.
If you are running it a LOT then once a year will be fine, if it is running only occasionally you will be fine once every several years. There is No carbon ever entering your compressor oil, the only contaminant will be moisture & things in the air, synthetic will deal with that fairly well... :smile:KennyV