If you look at page nine of the Mobil 1 0W-20 MSDS, the additives include
PHENOL, 4,4-METHYLENEBIS(2,6-BIS(1,1-DIMETHYLETHYL)-, which has two benzene rings in it. Probably mainly an issue post combustion, where the benzene has been broken out. I note that benzene is mainly mentioned as a used motor oil component issue, so contact would occur during cleaning of a bore that had previously been cleaned with this oil.
I understand from board member Hummer70 (aka, Humpy) that the Army Marksmanship Unit uses Mobil 1, having found it good for cleaning (carbon removal in particular-most motor oil will do this to some degree—just let it sit in the bore for awhile before patching out, as it is not a fast penetrant—but apparently the carbon detergent additives in Mobil 1 are superior). So if you don't have small kids around, the benzene exposure idea may not bother you. The lack of corrosion resistance is, however, plainly visible to the naked eye. The AMU weapons are not subjected to hunting conditions, much less combat conditions, so corrosion resistance simply may not be an issue for them.
For the sake of full disclosure, I should reveal that I have a quart of this oil that I got on Humpy's recommendation. As a former 2-time US Palma champion and former Aberdeen Proving Grounds Test Director and Incident Investigator, his knowledge is reliable. In this instance, though, knowing I have a good number of guns that see infrequent service, I am more comfortable having some degree of known good corrosion protection on them.
It has occurred to me before that
air tool oil includes corrosion inhibitors and might be a good inexpensive thin oil for firearms for that reason. These oils also get misted into the air around users, so I expect they don't have much by way of toxic compounds in them. Maybe not as good as food grade in this regard. The
Endura brand includes this in part of their description:
"…a unique combination of special base oils and additive treatments which virtually eliminate the normal rusting, gumming, and wear problems. They provide: emulsifications of moisture in compressed air, detergency and cleaning action to keep the tools clean and the air passages open, anti-wear, superior adherence, rust and corrosion inhibition and oxidation resistance."
They make several weights.
There are, BTW, some other interesting things out there. I've mixed Mil-Comm TW-25b with JB Bore Compound and a little bit of light gun oil to make a slurry. You put this mix onto any surface that slides and work the parts back and forth a bunch of times until becomes very smooth. This is a version of the old S&W armorer's trick for making double-actions "buttery smooth". I just caution you not to use the PTFE type products in a barrel. It probably won't mater to a pistol barrel, but in a rifle barrel the fact PTFE properties change with temperature has proven to cause POI to drift as the gun heats up.
I've used Shooter's Solutions Moly-Fusion, which is a thin conversion coating with a synthetic lube in the matrix that is really slick and waxy and permanent and dry. It does, however, darken the surface of steel, so you don't want this where it is visible. On aluminum is it white and clear and may be used without any apparent change in color that I can identify, but certainly makes it slick. I took apart and treated my whole Lee Hand Tool press with this stuff, and now dies just glide in and out of the top threads and the handle just falls open when I pick it up. It attached to the links without removing the black oxide finish, which surprised me. I will be trying it on some blued steel at some point, but haven't got the experiment organized the way I want it yet. It provides a small degree of corrosion resistance, sort of like super thin Parkerizing might do. Not great, but a little help.
Sprinco's Plate+ Silver is a NASA patent lube that bonds electrically to valence electrons on the surface of steel and stainless steel (does not work on aluminum) and has a colloidal suspension of micronized, acid-neutralized moly to provide a barrier lubricant that settle in the metal's microscopic surface imperfections over a period of about 3 days. You can wipe it as dry as you can, but it still lubricates. I've applied this product to a number of tools, like the steel parts of my Dillon 600 primer pocket swaging tool and the thing is so slick it falls open. The company's Machine Gunner's Lube is a higher viscosity oil that contains some of the Plate+ to build up a barrier lube over time.
Did you actually look at the results? Pretty typical of what I've seen from other tests involving motor oil, synthetic or otherwise.