Those have a ridiculously high failure rate even OE. I've replaced them on a 30,000 mile 3.0i and a 25,000 mile 4.6is.

A case could be made for age, but I've had older cars with higher mileage that had longer lasting boots.

I just count them as more "character" - building quirks of E53 ownership...and I sure have built up a shit-ton of character!

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04 X5 3.0i auto
03 X5 4.6is