Here are my notes from when I did it on my 2001 back in 2020. Tells exactly what worked (for me: 21mm ID x 2.5mm thickness = 26mm OD) and what did not work. Every half millimeter counts. It would be good to have a few choices unless you are sure you know what you will need.

The removed original o-ring seemed to be 23mm ID x 2mm thickness = 27mm OD.

I believe BMW spec is 22x2, so maybe mine stretched.

Replaced with one of the two spare o-rings from the engine work. I think it was for the VANOS solenoid. Measured 21mm ID x 2.5mm thickness = 26mm OD.

Slightly thicker seemed to achieve the desired improvement. A little silicone grease to aid reinstallation made it easy.

Initially tried and failed to use some Viton o-rings from a set bought at Harbor Freight. Biggest one: A2311 = 20.22mm ID x 3.53 mm thickness was way too big to fit. Tried A116 = 18.72mm x 2.62mm. That fit OK, but was maybe stretching the o-ring a little too much, so I used the VANOS leftover.

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In summary, the thickness was the critical dimension, since it needs to fit, and you want it snug enough to provide some structural support. I found 2.0mm to be too thin, about 2.5mm or 2.62mm to be just right, and 3.53mm to be too fat. The overall inner diameter is not so critical, as long as it is not too big. If too small, it can stretch a little.

I would not use "Teflon" tape without doing my own long term test on a shelf in a jar with gasoline, not knowing exactly what is in that tape other than PTFE.

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