Hi, it's best to use polyurethane.
Mr Lawrie Amatruda is very good at reasealing valve stems.
Lawrie uses a high quality polyurethane which will last for many many years.
He is a very well respected airgunbbs member.
An NBR o-ring will not work for the valve stem.
By the way, for the cap and valve o-rings, it is best to use polyurethane o-rings. They will last much longer than nitril.
Hope this helps.
Cheers, Louis