I looked at a diagram in my engine manual, and it looks like filtered oil flows through the pipe that goes through the ID of the filter and back to the pan. I suppose the o-rings keep dity oil from going back to the pan. The page i am looking at is 18.4-005/11 of the engine service manual for engines 615, 616 (240D), and 617.91

------------------
'82 300D Turbo 218k miles