This is a great topic, because I want to store my Umarex Gauntlet .177 away because I don't shoot it as much as my others, anymore. I have it all Hajimoto'd out to the max, so it's an awesome rifle.
I wasn't sure if I should store it with or without air. Kind of like a car - When storing a car for long periods of time, it is recommended that you drain all the fluids and have an empty gas tank. I'm not sure about airguns if it helps to have pressure on the O-rings, or not. Will pressure keep them working longer or is an empty PCP just a leaking rifle waiting to happen ?