Some folks use brake fluid to clean the internal parts of a gun. You can use WD40, but you MUST remove all of the oil before reassembly. You can use alcohol as a de-greaser.
If you have a leather seal you will need to use a silicon type oil and NOT regular oil. You do not want to end up with ANY combustion type oil in the compression chamber other than the silicon oil treatment for a leather seal. It is important to use only the amount required to prevent excessive dieseling for the initial break-in. As you probably know, if the gun has a leather seal, you can most likely change it to a synthetic seal in which case, you do not want ANY oil in front of the seal or in the chamber (the rear end of the seal can be slightly dressed with the appropriate lube to prevent burning the seal).
This is a tutorial that should help you on your project:
http://www.fortunecity.com/olympia/kickbox/377/xs-b20/xs_b20_index.html
Also check this website for the lubes you will need in addition to being a source for springs, guides, seals, etc.
Important: Specific lubes are best for certain parts of the gun.
http://www.airguns.citymax.com/page/page/251327.htm
Good luck and let us know how your project turns out...........