If you put an O ring under the head of the bolt, it will compress and distort when you try to tighten. This is just like any drain plug...they all seal with a metal ring just under the head of a bold between two contact surfaces. O rings are used to seal things that pull together without twisting motion. Metal seal rings are used were contact surfaces twist while tightening...such as with drain plugs, and this filter screw.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)