I don't know about the X5, but for the E46 3 Series, Beisan Systems has come up with an o-ring replacement kit for $60 which completely takes care of the problem. You might want check to see if they have, or are developing a kit for, the X5.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT