I have the same issue with my 420sel with only 59k miles. But my leak is not that bad, only few drops at the most. Level seems fine. I just changed the fluid filter and pan gasket. My mechnic says that there are seals in the transmission that are old and inorder for the leak to stop for good, I would need to have all the seals and gaskets replaced. The car is almost 20 yrs old bear in mind. Leak doesnt really bother me as long as I dont run out of fluid.

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6