Get O-ring size 22 x 2.5 mm (I.D. x thickness).
You can find it at "O-ring and more" website, Henrico, Virginia.

PS: iirc, factory is 22 x 2 mm. The difference of 0.5 mm in thickness makes a difference.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT