Would replacing the seal itself solve this problem? You said you still had a leak, even though the seal appeared to be in good condition, so it kinda sounds like a new seal wouldn't make things better. Both the SD and D have leaks in the same location.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.