How much mileage on the truck, and what kind of other previous AT repairs and maintenance (if any) have been done?

I have a 3.0i, which has a different AT (GM vs. ZF), so I can't comment specifically. But when I starting drain+filling my AT for the first time ever at about 169k miles (when new to me), the first fluid D+F (with pan drop, cleaning the pan, new filter) did end up knocking a bunch of new debris loose, which I found in the pan on the second D+F a few hundred miles later. I did the 3rd and final D+F a few thousand miles after that and it has been flawless.

So if you think that data point is relevant to your situation with your ZF, my point is that you can consider the new fluid to have solvents in it that may release new debris/problems just from doing the D+F.

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2001 X5 3.0i, 203k miles, AT, owned since 2014