Tar, at least the black kind, won't hurt the gun or spring. If anything it'll prolong spring life since it greatly dampens vibration.
The only way to put a gas spring in the guns mentioned is drastic modifications. For example the Umarex Octane is chinese clone of a 350 modded to accept a gas spring. If I were to attempt it I suppose I would hack off the Quattro trigger assy from a Hatsan and weld it to the Diana, along with the other mods needed to make it work. The reason for the Quattro rather than the Octane trigger is Umarex cut corners big time to make the conversion work and the trigger suffered the most. So not worth the effort or sacrifice of a good 350 and Hatsan imo, but it is doable. No kits I'm aware of, but would love to see one if you have a link. A coil spring is more reliable, cheaper, and adjustable. Once quieted down the nitro's got nothing on it.