I replaced that o-ring, along with the lower radiator hose (and a bunch of other stuff) recently on my '01 3.0i with 186k miles, and there were no problems. Plastic things un-snapped like they were supposed to, etc.

And BTW, my existing o-ring was fine, nothing leaking, but I put the new one in anyway. As with all those o-ring fittings, a little silicone grease or lubricant will help it seat better, which will help things seal better.

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