When I did my upper timing cover, there was an o-seal that went around the camshaft, but it was hardly an o-ring. It was a regular radial seal complete with lip, etc. ala main crankshaft end seal.

Is this what you're talking about?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
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85 W126 380SE Cranberry
79 W123 250
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75 W114 280