BMW dealers don't repair transmissions but they may know of someone that does or the local BMW focused independent shop will have someone they use.

As far as the question to change or not to change the fluid--there are strong feelings on both sides. BMW says the fluid lasts the lifetime. When they do the transmission work I feel sure they will replace the fluid. Assuming that is the case, you won't need to make a decision either way for at least 50,000 miles.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas