I think it is 015-997-94-48, 2mm cs X 6mm id, same as on the lower groove on the oil canister cover shaft.

To bring down your cost, just get a box of assorted Viton metric o-rings from the local auto parts store or Harbor Freight.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022