If there was anything adverse happening to firearms from the use of motor oil on them, you would have heard about it by now. Especially today with social media a tap or a swipe away from everyone's hands 24 / 7. It would be all over gun forums and blogs everywhere. There isn't, with the exception of a few negative opinions here and there. Which you can find with most any lubricant in existence.
People have been using motor oil, (both conventional and synthetic), successfully for decades on weapons without any ill effects. Either to the guns themselves, or the people who own them. Companies like Lucas Oil have produced, "Gun Oil" in every color of the rainbow at one time or another. Who knows what's in it? Their primary products are all automotive driven. So how does anyone know for certain if their "gun oil" is not simply a rebranded automotive product? It certainly sounds logical.
Mineral oil, Crisco Oil, (Fireclean), Ballistol, (scented Mineral Oil), "Frog Lube", (rebranded roller coaster track lubricant), and literally hundreds of other products have been successfully pedaled as "gun oil" over the years.
Most all of them being endorsed by some ex "operator", or Navy Seal who swears by the stuff. They hold the public interest long enough to sell for a profit for a while. Then the swagger wears off, and they generally fade into oblivion.
I think all of these products work to some extent. And none of them are technically "bad" for your guns. With the possible exception of Frog Lube, that has shown to gum up guns that have been stored for long periods of time after having it applied. Some will no doubt work better than others. But I don't think any of them, least of all motor oil, will damage them. Or cause their owners to book an appointment with Cancer Treatment Centers Of America.
All of this "gun oil" business is much like Baskin Robbins. They keep their business fresh with a new, "flavor of the month", that everyone likes to try..... At least once.