We have a W34, which came with what I was told are the standard issue gas valves. Every valve on our engine wore out, to failure, at just over 4500 hours run time- worn to the point that they had nearly double the allowable internal clearance. The cause of the failure was blamed on our fuel gas, that it was too “clean and dry”. We have pipeline spec quality fuel gas. After the failure, we have replaced all of the gas valves with a new style, that has a coating on the wear surfaces (sorry, I do not know model numbers on the valves). I plan to pull a couple of valves to inspect at our next major service, to see how they are holding up.