The owner’s manual that came with your pump will likely say that you need to change it after each use. This does not apply to us who are using it to vacuum form holsters. They operate under the assumption the pump is being used to vacuum HVAC lines. When you first receive your new pump, it will come with the oil in a separate container. (Here is a valuable tip: Before you fill the pump with oil, draw a line with a marker at the oil level on the container. Do not discard the container. Save it and use it to change the oil. You will pour oil into the container first and measure it instead of trying to guess when the pump is full of oil. The sight window glass takes awhile to adjust and is not immediately accurate. Fill the pump with the oil. Let the level adjust and confirm that the oil level is in between the Low and High marks on the sight glass. The oil will be almost clear to crystal clear when you pour it into the pump. The more you use the pump and the more humid environment in which you are operating it will dictate when it needs changing. Your vacuum achieved will degrade over time as the oil degrades. We usually change the oil in our pumps when the oil starts to turn a light yellow or light honey color. It is cheaper to buy the oil by the gallon than the smaller containers. You may change the oil more frequently if you are in a humid environment, dusty shop, etc. Do not wait to change the oil until it reaches a dark honey or pancake syrup color. Also, it is best to locate your pump on a shelf so that the oil drain on the bottom of the pump extends over the edge of the shelf. This makes it very easy to change as opposed to it sitting on top / back of your work bench. Never ever use motor oil in your vacuum pump. Use the oil recommended by your vacuum pump manufacturer.