The differential vent I'm talking about attaches to and parallels the dipstick tube in my 2001 3.0i.

The main distinction of this vs. the other hard plastic tubing you'll find is that it terminates in a constant radius 180* u-turn. That sort of an end is not so common other than for applications like this.

I expect searching on this site for that will give some photos.

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