Originally Posted by BloodSport
It's not so much keeping gas in them as keeping them lubed. As long as you add oil to your propane when filling as suggested, then your seals will stay fresh and not dry out.
Thats the main reason for keeping some gas in the mags if your not using them for extended periods.
That won't help at all with the primary o-ring that is inside the mag that will dry out eventually. Silicone oil will only lube the valves.
Keep some gas in your mag, just enough to keep pressure on your primary o-ring in the bottom of the mag.