For some reason many seem to want to discount WD40 for its ability to lubricate. IMO - it is pretty much a miracle chemical that comes in a spray can and does a lot of good things in the mechanical world for the professional "Tinkerer".
It lubricates, can loosens rusted hardware, cleans road tar from paint and many other uses.
I suspect the problem with the valve may be more related to the handle/cable rather than the movement of the valve. They are difficult to lubricate once installed. If you have the cable outside the rig (not installed) try to drop oil into the end of the cable where the inner wire enters the housing.
http://wd40.com/about-us/myths-legends-fun-facts/