It doesn't matter what hardness is used in this application. If the valve is screwed down properly and the right sized O-ring is installed, it will seal. The soft green ones that are being referred to here are fine for this use. the problem with them is they can only be used once. After that, they will be distorted and like airgunfans says they will not seat properly and pinch slightly and stop the interface butting completely allowing enough gap for the rubber to squeeze out of. From your picture, it is impossible to know whether your threads are metric or imperial. It should be printed on the side with the other specs. If it is metric, use the size I recommended. 18x2.5-2.75. If not, look it up or measure what was supplied and use a slightly smaller cross-section one and tighten it until there is zero gap. The recommended torque is 70 FP but it doesn't need to be that tight. I said it before and I will repeat it now in case you didn't understand. If there is no gap there can be no blowout.