I would be careful of the "flush" step. In a garage, they pump generic atf through the transmission. I've never found a place that does this with the $20-$40/liter synthetic fluid specified by BMW.

I did drain, drop pan to change filter, cleaned pan and refilled with Pentosin ATF1 at 135k miles. Repeat if you want to change a higher percentage of the fluid. About 2-3 liters is retained each time. Mines been going strong for another 10k miles since. Definitely seemed a little low at the time.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles