Hey Picclock,
I would be loath to suggest an o-ring for this application. I was a member of the team that wrote the o-ring section for the 1976 edition of SAE's
Automotive Engineering Handbook. (You can see my resurrection of this now "lost knowledge" at
http://www.scribd.com/Lew%20Merrick.) I would consider a labyrinth seal as seeming more appropriate to your description (30,000 rpm). An axial clearance in the .040/.030 range should be sufficient and a diametral clearance in the .010/.005 range (at temperature) with 3 full "blanks" ought to get you there. I would (speaking through my hat as I do
not know the details of your application) make the shaft side a one-piece, press on detail and the housing side a two-piece clamshell attach assembly. Given your rather low delta-P (200 psi), I would consider using a round-plate-with-hole stress analysis to set the thickness to (say) nominal to yield at 450 psi. (Roarke's
Formulae for Stress & Strain would be the most common source, but I prefer Blake's
Practical Stress Analysis.)
Once the air inside the labyrinth goes turbulent, you will have very little leakage. ???