This is a quality made O-ring made from Nitrile N70 which is a durable rubber.
Nitrile is an ideal sealing compound 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.