I decided to dig into a Mares Frontiersman that doesn't hold air and had been sitting around for a few years. It's the same length as the gun I use, so I wanted to prep it for a backup. I used silicon oil a few years ago in a couple of guns, but in looking at a Mares service manual, it says to use SAE 10 hydraulic oil, like Mobil DTE Light. From what I can tell, that's a mineral oil.
Which should be used, silicon or mineral? The o-rings were so hard at first I thought they were plastic - could that have been the choice of oil that was used?