I cant comment on the X5 yet as mine has only coverd 18k, but on my E39 540, I had the gearbox serviced every 25/30k and when I sold it, there was 130k on the original transmission and it was running as smooth as the day I first got it.
Not sure if there are many BIG differences between the two and if the X being 4WD is a bit more sore on the transmission but its still a service I'm going to maintain on my 4.6 and hopefully wont run into the problem.

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