Just had repair done on my transmission oil gasket due to a slow leak. The oil was dark red(dirty). Not sure if we are talking about the same oil(maybe it has nothing to do with your issue), but I believe that the work order mentioned transfer case gasket??? Anywho, BMW fixed it last week under CPO program.

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2011 X5 xDrive50 Black/black////M sport pack/all options
2006 ///M3 Cabrio Gone but not forgotten
2003.5 ///M3 Gone but not forgotten
2007 Aston Martin V8V Gone but not forgotten
2007 Porsche turbo Gone but not forgotten
1995 540i M-sport(1 of 70) Gone but not forgotten