Bite the bullet and go for the factory key. A lot less trouble in the end, guaranteed to work, and you have a key with a new battery if/when your original dies. I just ordered a second key yesterday myself. I'm pretty cheap but in this case not worth the headache.

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