There is a lot to be said for getting the X back that you bought. I don't think you made a mistake and the purchase prices they offered were not realistic to your situation. Now that they are going to rebuild your engine just ask them if they have checked downstream of the engine such as the above and verified the oil didn't do further damage. Have they provided anything for you to drive while the X is in the hospital?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas