Not saying it is not, but I am not convinced the problem is, valve stem seals. One reason is that you are not seeing oil smoke that often and even then it doesn't sound like it is severe. Another is that only one cylinder is problematic. I would be sure that valve seals need to be replaced, at least get a second opinion from a well recommended BMW Indy mechanic.

I think it is better to stay with the engine you have rather than purchase an engine without any history. Almost always better to deal with the devil you know rather than the one you don't know.

First I would have the engine checked for any error codes. Depending on the findings, I would have a compression, leak down and smoke test done and go from there.

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