So what happens when you start to fire(use) the firearm?
If your firearm runs on gasoline and you go 5 million combustion cycles before cleaning and re-lubing, you might have the same issue, yes. If not, unlikely.
Used motor oil is carcinogenic because it absorbs partially unburned combustion products of gasoline from piston ring blow-by, then steeps said combustion products at a couple hundred degrees, with added water, for months. If you filled a crankcase with gun oil and ran it 5000 miles, the gun oil would contain the exact same carcinogens. Neither gun oil nor motor oil is carcinogenic fresh out of the bottle, though.
I get it though. Motor oil does lubricate moving parts as that's what it was designed for... In automotive engines, but regardless it lubricates right? Just keep in mind that it is not optimal and was not designed as such.
Oil has physical properties, not metaphysical purposes, and for *any* use you pick an oil with the best physical properties for that application. A firearm---or a car---doesn't see the label on the bottle; it sees only the physical properties of the oil.
I use Mobil 1 EP 5W30 because its physical properties (film thickness, viscosity, shear strength, temperature curves, additive pack) are in my opinion about as good as you can get for a general purpose gun lubricant, and due to those same properties I again was already using it in my J32A3. I don't use it as an exterior preservative, though, because the properties that make it a good lubricant (film thickness and persistence) make it messy on the outside.
I'd challenge you to compare the physical properties of (say) Rem Oil vs. Mobil 1 EP sometime, and then tell me how Rem Oil is such a superior firearms lubricant because it has a gun company's name on the label.
Remington and other gun-oil companies don''t own any refineries; they buy oils and additive packs from oil companies based on the specs they want, blend them (or have them blended), and market them as gun oils. The blend they come up with is often a compromise between exterior and interior use, which is why Rem Oil is a better exterior coating and preservative than Mobil 1 (because it is thinner and more volatile), but those qualities make it somewhat less robust as a lube.
Regardless, I cleaned a used g26 a few weeks ago that had motor oil used on it. It took me disassembling the entire pistol to get that crap out of it. It was everywhere. Just a magnet for dirt and carbon.
What kind of motor oil? They aren't all created equal.
Having said that, a good gun oil is *supposed* to be a magnet for carbon; it dissolves carbon (or should) and thereby keeps it from accreting into carbon cement on your working parts. It will also keep fine dust fluidized so it doesn't accrete and jam the action. Those two characteristics of *any* good gun oil are why AR's run better wet than dry when they are very dirty. And in my experience, Mobil 1 definitely stays put far longer than most gun oils I've used. I consider that a feature, not a bug. It also makes cleanup a snap; just wipe the bolt, BCG, and receiver with a cloth and re-lube, or (in a pinch) just add more lube to flush the accumulated crap out.
Having said that, I wouldn't use a non-synthetic motor oil on a firearm except in a pinch because fossil-derived oils smell and are inferior to synthetics, and I can see the hassle of trying to get a brown and stinky distillate oil (if that's what you were dealing with) out of a gun's nooks and crannies.