What would you use as a lubricant, aluminum to aluminum xmas tree gas valve.

I never had such an application, but I have found Sil-Glyde from NAPA a good solution to many lubrication problems. It is an inert silicone grease safe for all metals, elastomers, and fluids except maybe oxygen. It never stiffens up and is resistant up to 600 degrees F. It is easy enough to find. The common 4 OZ tube may be a lifetime supply. Sometimes you can find small packets of it.