Pretty much any oil will prevent rust if used correctly. All I know is that I had a terrible time loading my BP rifle until I followed the advise of those who knew better. I started using Bore Butter inside and out. Since then I never have any trouble loading, no matter how much I shoot, and I never clean between shots.
The theory, as stated elsewhere, is that petroleum products react chemically with the powder residue and form a harder fouling, which can lead to loading and functioning problems, and which makes for more difficult cleaning. I don't know, but now I keep the BP guns slopped up pretty well with bore butter (and olive oil as a lube on the revolvers) and they load, shoot, and clean up really well with no fouling problems and no rust. I fired about 50 rounds through each of my .44 revolvers yesterday and they never got hard to operate.