Pretty much any penetrating oil seems to be adequate for the typical pellet barrel. WD-40 or Ballistol as mentioned above, Hoppes #9, Shooter’s Choice, Kroil, Goo-gone, etc.
I use Hoppes these days but have used most everything on the list and got satisfactory results from them all.
One of these and snug-fitting cotton patches is usually all that is needed to keep a pellet shooter clean. Contrast that with a long neglected and heavily leaded bore or choke, or a rough bore, or shooting slugs, or lead with high antimony content, or some combination thereof...something like that may need the scrubbing action of a bronze brush. It will not scratch a steel barrel, just do not use one on a brass barrel.