Unless the transmission is leaking there is nothing to top off. I think the majority of the horror stories about changing fluid causing transmission failure are from those that tried to solve an already existing problem with new fluid.

I spent quite some time with the techs and owner that work at a transmission shop that has done nothing other than European cars, primarily BMWs for twenty plus years. His input was to change fluid and filter every 50,000 miles and use only the brand fluid and type fluid that is specified by the manufacturer. I didn't ask if he would recommend changing the fluid after two hundred plus miles. If in your place I would have a well recommend transmission shop that works on BMWs diagnosis the problem. Parts guys or BMW dealers are not transmission experts.

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