Either air or Nitrogen will work. The manual only states air, not to exceed 12psi for the XL, if memory serves me. I´m 1400 miles away from my manual at the moment. For damping fluid, Fork oil such as sold by Torqo or automatic trans fluid labled DEXRON will work. If you do use ATF, MAKE SURE IT IS LABELED DEXRON ONLY! Cheers.