Compressor oil. You can get it at an auto parts or home improvement store. You will want to contact the manufacturer to find the correct ammount to add.
In that case, use straight 30 or 40 WT oil, not multiviscosity like 15W-30W.
If you are outside in the winter, i.e. -10 degrees, consider 20 WT.
Hope that helps, oh by the way, DO NOT overfill the unit, you could damage it. Look at the dip stick or level indicator
You want ND-30 oil. It is a 30-weight non-detergent oil used for small engines. The cleaning additives found in regular motor oil can be bad for the small engines found in lawn mowers, generators, and such.
I know wal-mart sells it in a black quart bottle with green label in their automotive department. They may also carry similar oil in the garden area with the lawn mowers.