I have a '03 330i and the FelPro 35489 was definitely too thick, it measures .137" (3.5mm). I could tighten the DISA valve all the way and the plastic was flexing so I could still see the O-Ring. The valve was not sitting flat. I am surprised no one has cracked the tabs off their DISA valve. There is an O-ring/seal store right by my shop so I picked up a 55mm x 3mm silicone O-ring for less than $2 and I would not put anything thicker. You can fit anything from 50 to 55mm. I also picked up a #136 Silicone O-ring (136 to 138 are all good) and that seems to seal it fine. That measures .102" (2.6mm) but I wanted to see what the thickest o-ring I could fit without breaking anything.
Make everything as simple as possible, but not simpler. Albert Einstein