Never use oil or any kind of lubricant inside your barrel. Lubricant should stay between your barrel and hop up rubber, and not get inside where it will contact a BB.
Clean with a cotton patch on a regular basis. Once or twice a year you can clean it with a cotton patch and Windex, a strong solvent like acetone or alcohol is overkill unless you have glue in your barrel, and can also damage the finish on cheaper coated barrels.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN