Google the model number, download a copy of the owner's manual, follow directions. Try searching that model number at harborfreight.com If the manual does not exist anymore, and the pump has a sight glass, fill with clean, non-detergent straight 30wt motor oil, available at most car parts stores. If there is no sight glass, drain what is in there and pour in 1/2 qt. No promises that is right, but it is likely close. Last option, go to a Harbor Freight store. Look for a compressor with a pump that looks like yours on size and shape. look at that manual and use that amount of oil. Good luck