I use silicone grease on the tank threads but moly paste would work just as well. I've replaced fireman's valves on SCBA tanks and they can be a bear to remove after years being tight with no thread lubricant. The large o-ring is the pressure seal and lubrication makes for easier removal later on.
Moly can work, but the best thing to use - if you have it - is Krytox grease. Lubes the o-ring perfectly, and insures there will be no galling and easy future removal with no issues of petroleum in an HPA environment. It is expensive stuff, but a little goes a long way . . .