The last time I certified as an inspector with PCI, they said that no matter what you do to a K-valve, it cannot truly be approved for oxygen service because of the number of turns the gas must take. They didn't offer a solution other than to realize that limitation when you approve a cylinder as oxygen clean anyway.
When the dive shop where I was working bought some Thermo isolation manifolds and prepared to set up doubles, we noted that the information that came with them said that they can never be used for mixes greater than 40%, no matter what you do to them. Since we would be making trimix through partial pressure blending, that was a problem for us. We called, and the Thermo guy confirmed that they could not be used for greater than 40% and, in fact, no such valves could be used for greater than 40%. As he talked, you could almost hear the CYA winking.
When the dive shop where I was working bought some Thermo isolation manifolds and prepared to set up doubles, we noted that the information that came with them said that they can never be used for mixes greater than 40%, no matter what you do to them. Since we would be making trimix through partial pressure blending, that was a problem for us. We called, and the Thermo guy confirmed that they could not be used for greater than 40% and, in fact, no such valves could be used for greater than 40%. As he talked, you could almost hear the CYA winking.