I tried out viton o rings and they tend to be too stiff.
Buna N works well and handles fuel just fine.
I like to use a Molykote o ring grease one them, but any grease should do fine.
The ones I currently use last dang near forever, but I change them whenever I take a tube off because they are cheap.
Oil-Resistant Buna-N O-Ring, 1/16 Fractional Width, Dash Number 010
I tried dozens of different ones and settled on these being my favorite:
https://www.mcmaster.com/9452K18