I'm not claiming to be a guru of any kind, but have meddled in O-rings a bit. I'd stick with Nitrile over Viton but check the manufacturers Parts list.
Viton is superior in high temperatures and Chemical resistance but a bit less of a natural sealing material. Temperatures are nowhere near high enough so only if the hydraulic fluid is very aggressive would I contemplate Vitons.
Common sense was never common
28h silver rear Suntour hub
32h Ritchey champagne WCS front hub