Tuning JDS air tank o rings

If you are referring to the O-ring on the end of a paintball tank bonnet, a -016 or -015 is what you need. No particular preference for one size over the other...the ratio of the diameter to the cross-section is so large that either works. Buna-N or polyurethane in 70 durometer or 90 durometer, again with no compelling reason to prefer to one over the other.