FWIW, I read on wikipedia that hydrometer type testers work by relating freeze protection to the density or concentration of the antifreeze, but that different types of antifreeze will have different relations between density and freezing point. So, they may not be that accurate with the special types.

An idea: If you have some of the BMW antifreeze sitting around, you could mix up a small sample with the correct proportion of water to give you the concentration that you ideally need, and see what your hydrometer tester reads. Then check your BMW and see what its antifreeze reads on the tester. If it's showing less protection, then drain some out and add enough of the good stuff to give you the "ideal" reading.

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2009 X5 3.0
2001 X5 4.4 - Sold