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.