I've only being rebuilding air rifles for a year or so as I got bored with firearms and therefore follow the advice given in the old Abbey Maintenance kits instructions, LT2 on new mainspring, Moly Paste on and off the piston head, SM50 on piston body. If there is still a twang from the mainspring on firing I will take apart again and put a heavier open gear Moly grease on the spring instead of the LT2. The instructions also say to put a few drops of Silicon Oil 35 into the compression chamber before reassembly but I don't due to all the stories you hear about this stuff. I've also found some Rocol Dry Slide which I buff on and off the cylinder walls but on my next gun I will be using fine graphite powder in the same way, buff on then off just to see if it performs as well as the Dry Slide. The last gun I did with a leather washer (Webley MK3) I soaked the new washer in Neatsfoot oil for a day after sizing it to the cylinder. I did however remove as much of the oil as possible before fitting by squeezing between kitchen roll. So far no problems. I'm also very interested to learn what others are using.
Remember, it is the strongest character that God gives the most challenges.