Oops. I have a feeling you'll have to go to the dealer. The auto parts stores I see on line generally have oil filler caps -- where you put the oil in -- but not the cap for the filter.

Once you get it, check your local Advance Auto Parts for a filter cap wrench that you use with your socket wrench. There are various sizes, with and without fluting; you'll have to check to see what size fits your (new) filter cap. It cost me about 9.95 or so, as I recall.

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-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino