"A light coating of motor oil should be fine to shoot on in a barrel. I've done it a lot."
Oil of any kind in a gun bore rarely harms anything, directly. Yeah, if it's heavy it will raise pressures a bit but not drasticly, IMHO. BUT, any such oil, even a wet layer of Hoppes 9, WILL cause the first two-three shots to hit outside the group.
I was told by a trusted rifle engineer (Mike Walker, formally of Reminton and developer of the 700/40x series of rifles) that if we clean a bore with a wet patch of Hoppes before shooting, and then run two or three DRY patches through, the first shot will usually strike in the group. I've found him to be correct.
ATF is a great LIGHT gun oil. I use it for short term - maybe a year - rust protection of any iron/steel surface. ATF is a good light lube for revolver actions and rifle triggers because it is a high penetrating oil that's non-drying (well, mostly, any liquid will eventually evaporate) and is non-gumming when it does.
Mobil 1 is very good for lubing auto-actions in rifles and handguns or for riding lawn mowers, etc. I keep a squirt can full in my shop. It's inexpensive in the quanities I use it, it provides good lubing in tight fits, good adhesion to the metal and is fairly resistant to water. Any dried residue (gum) can easily be cleaned with most solvents, including ATF.
The government does buy from the low bidder but the low bidder doesn't get to choose what it will sell for that low bid. Any item, including any gun care product has to meet preset MIL standards, and they ain't low standards!