Seeing an oil change thread bump reminded me of this.
My parts diagram shows 2 rubber o-rings which seat against the oil filter - one on each end. My bike came from the dealer with only the one that seats the inside of the filter... the one that goes between the filter cover and the filter (#19) wasn't installed:
When I took it to a "dealer" for its first oil change, he had no filters or o-rings or gaskets of any kind. I happened to bring a filter I got from my purchasing dealer, so the filter got changed and all the o-rings got reused - which means still no o-ring between the cap and filter. Then I ordered the 5-change oil kit from CMW, and they didn't have all the o-rings, so my first home oil change also saw no o-ring.
By the time I was ready for my second home oil change, I had the o-rings, so I installed one and found it a challenge to get it to remain aligned between the filter and the cover's pressure plate (my 2011 model came with the modified cover whose components are assembled into a "one-piece" unit). It was after this oil change that my oil level inconsistencies began. I took the o-ring back out, but nothing changed.
What's the deal with this o-ring? How important is it really? I notice no difference in engine operating noise or temperature whether it's there or not...