This interesting article turned up in the last issue of the US AOPA magazine
Most people thought Viton would be good for everything, but probably not when we get “paint stripper” 100UL like the Gami one 
What does “age controlled” mean?
Interesting. I googled it: it has a defined shelf-life, tracked by its cure date (the date the rubber was molded).
So it actually means the same as “age limit”?
I don’t know. Maybe an age limit which varies, whereas rubber tends to be ~5 years.
I had a kit of rubber o-rings which are hardly useful for aircraft so I went looking for Viton ones. They are readily found on Ebay, either brown or green. Green is said to be more common in Europe.
Fluorosilicone I cannot easily find in kits.
Seeing what hassle some people have with seals, I think a kit would be very handy – even if a lot of the time the swap has to be done off the books.
The main “legalistic” issue is whether the component has a CMM (component maintenance manual). If yes, an appropriately qualified person can change the o-ring. If not, you have to buy the whole component. And some are unobtainable. Part 91 (N-reg) appears to offer the concession described in the PDF. Not sure if Part-ML has a similar concession, but some cases I am aware of (long AOG) suggest it does not, or at least people in the maintenance business are not aware of it.
