I know some people who use synthetic motor oil to lubricate the moving parts on their AR's, so that part doesn't sound too strange to me. As mentioned before, motor oil doesn't have anything in it that will cut copper or lead fouling, and it will be marginal for removing carbon. My personal preference is to clean with a good solvent like Hoppe's #9 or KG 1. I prefer to lubricate with a quality CLP, but a little Mobil-1 will do the trick for that, too.

I'm sure product specific cleaners do a better job, but liquid wrench sure loosens carbon well. And brake cleaner flushes the Liquid Wrench and old oil right off. Then CLP finishes the job.

I've always been a little leery of possible, wood finish, plastic or rubber problems, never had any.

Really humid here, a nice coat of oil is necessary. Bolt guides and firing pins are the exceptions.

I have a couple of cans of really old Mobile 1 around in 5 weight (forgot to read the label when I bought it). Wonder if it is still good and how it would work? Synthetic motor oil has been known to soak into automotive seals and make them swell a little, likely not something you'd want to use on a auto loader with "O" rings.

ATF has even more detergents in it. I've used it on M-73 machine guns, the only thing that seemed to work. The M-73 is notoriously reliable, we tried everything, ATF worked the best.

I use remington Brite bore on all my handguns and rifles then followed by rem oil never any issues with cold gumming or rust. My muzzleloaders I use a custom blend of water soliable cutting oil,simple green,hoppes #9 and water I use it for cleaning and patch lube at matches too. then I use butches gun oil as final patches and that oil gets the rest of the barrel real clean.