The o-ring is easy to change and will not be that hard. It is available separately from the sensor. There is a chance that part of the problem is that the plastic housing portion that it sits in is slightly warped though as well. The teflon, as andrewwynn suggested, works well and is really quick as you don't even need to remove any coolant if you let the car cool down and don't take off the cap.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)