I have been very satisfied with Motorcraft bateries. These are made by Johnson Controls, and are identical as far as I can tell, to Interstate batteries. At under $70 I can't go beat this.
see:
http://www.motorcraft.com/standard.asp?T=2&M=3&PageID=22
You will want to avoid anything made by Exide
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I like the specs on the Optima, and the fact they appear to never leak, so battery trays should be happy with that, but the size/footprint and the post arrangement seems to be something that leaves some adaptation necessary if you want it to operate properly and not short out or something. The Motorcraft group 49 fits perfectly and come with the vent tube that can be used to drain excess battery acid away from the battery tray, in case it is overfilled. I believe the BMW requires this drain tube because batteries are situated inside the vehicle.
DDH