If there is a rubber seal involved, ATP AT-205 is specifically designed to deal with that problem. I used in the crankcase of my m54 to basically stop the oil leak that was "getting out of control". (I can wait until the weather is nice to find and fix the leak now).

Before rebuilding a transmission, I would definitely try at-205 first.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)