[AKnowles]

I would not leave it on as it could prevent a good seal in the future, and if you pry it off be sure not to score the surface where it seats or you could have the same problem. As an FYI, make sure you are looking at the oil washer as the sealing area is clean and looks like a washer (reflected light). The PO or prior oil change may not have even installed a washer. If you are sure it is a washer, it should pop off with little force. I found my first washer on the garage floor after having an issue like yours (turned out the area I thought was the washer was just the seal are).