This is a quality made o-ring made fron Nitrile N70 which is a durable rubber.

Nitrile is an ideal sealing compount for post o-rings as it has excellent gas barrier properties and also has good wear resistance so it should last a long time with the disconnect slipping up and down the side of it.

Losing gas can be expensive and changing o-rings is cheap.  For this reason it's always good to change this cheap o-ring out periodically.

These are the o-ring seals that fit on the top of your ball lock and pin lock kegs.  These handy o-rings also fit garden taps too so you can stop that annoying leaky garden tap a the same time.

These are a very common o-ring type and definitely handy to have a few spares.