What you just described is exactly what the dealer is claiming the leak is now. They are back tracking from saying it was the alternator bracket gasket leak (because there is not one), and now back tracking that it is the upper time case gaskets. David X5 has an old thread about that I did a post.

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2001 E53 X5 3.0 Auto. Silver Black Build date 10-17-2000 141kmiles

2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles