I would not be worried about ATF contamination there.

On the SAE vs. metric size, what lots of sets do for things like this (where it's an imprecise size to start with since it's not really fitting on the screw head) is to make one set that is "close enough" for both SAE and metric. I think that Craftsman set I have has a piece that is supposed to be 17mm, but it also has one that is supposed to be 5/8"-16mm ("close enough"). Another nice thing about the Craftsman set is that it is impact worthy.

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2001 X5 3.0i, 203k miles, AT, owned since 2014