Thank you gents.

@Ped, those photos are super interesting.
I have just changed some cap o-rings to urethane lol.

There is still the question why I sometimes find silicon o-rings in for instance old, factory original Sharp CO2 guns, and o-ring kits.
Would be interesting to know. I guess the answer is: it works, but it can damage quickly, so urethane is better.

Will a nitril o-ring, say nbr90, leak CO2 after a while, when it is for instance used as a valve body o-ring? After is has deformed under the influence of atmospheric pressure.

Lastly, another area of interest is valve seals. Here are different materials I have in my spares box.
The red polyurethane one, made by Lawrie, seems to be the best, in the fact that my CO2 guns with Lawrie's valve seals have never developed a leak so far (touch wood). With the orange TR Robb seal, which is quite soft, my QB78 started to leak from the valve after about 1 year.
I also regularly encounter translucent, colourless valve seals.