Simple automatic transmission fluid works fine. Need no more than enough to produce a film on the nose of the cartridge to prevent it from sticking to a valve's face seal. CO2 guns that pressurize the whole tube (instead of just the internals of the valve) need no lubrication.
When I ran out of secret sauce, I mixed up some non-detergent 80/90 weight gear oil and Honda brand 10 weight fork oil. At a 40/60 ratio it's been working great. The gear oil provides lube for the moving parts and fork oil is great for the seals.
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