Can't seem to get a M103 rear main seal to last more than 4-5 years. Bummer.
I really think the leak occurs around the seal cover/block interface, not the seal ring.
I have always used Permatex black. Maybe not the best thing?
I also use Permatex black on the front timing cover, and it also lasts about 5 years.

Did a thorough search on the sealants, but still confused.
The cover is metal-to-metal, so may need an aneorobic sealant?
But the base of the cover rests on the pan gasket, so needs a conventional gasket sealant to mate there?

The M103 book says 001 989 45 20 10, but that's probably been discontinued for 20 years.

Gurus here recommend Loctite 5900, which I think is aneorobic - but I have had zero luck finding it, except maybe by the case for $300.

Others say the new Hylomar formulations are good, or the current MB 003 989 98 20 10; but I can't find out much about it (except it's $$$$).

Any thoughts?? - don't want to do this again for awhile.

Tks,
DG