I truly don't mean to insult you with this answer, so no offense intended with this answer (please tell me if I'm missing what you're asking)... that oil fill cap is to put your 2-cycle oil into the oil tank. The motor in this jet ski is a 2-cycle motor which means it burns a gas/oil mix when it runs. Without the oil, you will burn out your engine -- fast. The oil pump and carburetor mix it for you while the engine runs, but you need to have oil in that tank along with gas in your gas tank if you want it to run.