Most Stanley-Bostitch compressors - portable compressors have a small drain screw at the bottom / middle of the crankcase cover. The crankcase cover is the plate that the dipstick inserts into. To drain the oil, remove the drain screw and dipstick and allow the oil to drain until the oil no longer flows. (Most Stanley-Bostitch portable compressors have approximately 6 to 7 ounces of oil so this process will not take long). Replace the drain screw (be sure not to over-tighten) and add fresh oil in the top of the crankcase at the dipstick hole. Check the oil level with the dipstick often; it is full when the oil level reaches the notched area on the dipstick. If too much oil is added, it mut be drained to proper levels.