Barring any guidance from the mfr, I would recommend you use silicone grease. It's cheap, readily available, and effective. Most importantly, it's safe for o-rings and is sometimes sold as "o-ring grease" in small tubes. Use the paste/tube rather than the aerosol spray. This, of course, requires some disassembly of your pump to access the working end of the piston.

I suggest you avoid all petroleum lubricants, including lithium grease, for your application. They tend to degrade most o-ring materials. They can also cause the weakened o-ring to swell and quickly wear against the wall of the air chamber, necessitating early replacement.