This is baffling me. I changed the wife's oil tonight. Everything went fine, nornal oil change. When cleaning up I noticed the rubber ring on the old oil filter was missing. Looking back the filter was tighter then usual getting it off. When I took the old filter off I wiped down the surface lilke I always do and didn't notice or feel anything unusual. I have searched high and low for the missing O ring to no avail. The truck is running fine and the oil pressure is fine. I got under the truck whille it was running to check the filter. It appeared to be seated fine and there was no sign of any leakage. The only thing I can think of is the old ring was stuck to the filter mount and the new ring seated against it. Is this possible and is it dangerous???
I really don't think a seal against a seal as such is safe. If indeed this is the case, it may last a long time or leak at any unknown moment. The design is for one only. I would certainly check just to ease my mind.
I had this happen on my Generator. It will eventually vibrate loose. and you will loose all your oil (at least a very high chance). Thankfully my generator shut down at loss of oil pressure and I didn't damage anything as far as I can tell. But in a car/truck, it's a whole different matter.
I will not put on another oil filter now without checking the old one first.
I had this happen as well on an older mustang and like others said, I lost all my oil within a week. Scared the hell out of me because I started it up one day and it just started shooting oil out underneath in the garage. Shut it down real quick and couldn't figure it out because oil was everywhere. So guessed at it and when I spun the filter off, there was the old and new rings double stacked. I was happy to dump another $40 of oil and filter into it vs what else I thought could have been wrong. So the good thing is that you realized it. One thing is, you will always double check now going forward!