Moly grease is what you would use. Not too much, just a film on piston behind seal, on the spring and the rear guide. I've used CV joint grease diluted with airgun oil for sliding surfaces and undiluted as "spring tar". You have to take the gun to bits to do that though. Why not just drop the action out of the stock and see how well (or badly) it's greased now? If OK, leave it until you want to go the whole hog, if dry, get organised for a complete strip, de-burr re-grease etc. The main thing with lubrication of spring guns is better too little than too much.