the 30wt non detergent is fine, otherwise get "air compressor oil" bottled as such.
As for how much it takes; If it has a sight glass in the pump, fill it so the oil is half way in the glass. If it only has a fill plug and drain plug, fill it so the oil comes out of or level with the fill plug. a picture of the pump would help, if you can do it.
Most compressor's that size will take around a quart of oil, but dont over fill or under fill it.
You can also try and search for the 6hp 60gal compressor on thier website. You may find a similar or identical compressor with specs you can go by. The tank should have a tag on it but that wont likely help with the pump.