The "O" Rings are definitely available separately. My local Dealer gave me half a dozen for free and suggested I replace them every 3-4 oil changes. Top & bottom are the same by the way.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown