How much pressure is suitable for storage ?

Whatever the gun reg pressure is set at or if not regulated to avoid any possibility of regulator creep then if not regulated then 3000psi max is what I normally leave and that's the max I fill to anyway regardless of how high manufacture states to have less problems with leak and weird regulator behavior unless it's a Benjamin Maximus that's 2000psi max.

20bar is too little air to leave in IMHO.

Putting away for winter I always leave 1500ish psi in the gun , put a lil piece of tape on the gauge then when ya take it out you can see if it’s leaked at all. Never leave mt. This is just what I have done for years. 

I keep most stored near full pressure.

From an O-ring longevity standpoint, the only reason to store at a lesser pressure is if the O-rings are expected to suffer extrusion failure. And if that were the case, it isn’t the proper O-ring for the job. 


Mind you, nothing wrong with storing at a lower pressure—as little as 200psi or so will keep the seals formed—but also no real reason to go to great lengths to do it either. 

Basically, anything but zero....

If I am not planning on shooting it for a while:

1) If it's a regulated PCP, I always store them about 200-300psi above the regulated pressure - this is so the entire system and all the O-rings remain pressurized during storage

2) If it's an unregulated PCP, I just store them about 60-80% from max psi to keep the entire system and O-rings pressurized

And I also throw in a couple of desiccant packets in the the storage case.

150bar-200bar 

My Rapid 12 I've had for 16 years has always been stored at or near 200b, I doubt if it was shot once a month on average, it was sealed twice in that time and the 2nd time was for up grades and wasn't leaking, I usually top off my guns after a shooting session to be ready for the next shoot never had any issues.