Check the auto parts store. I often see it there when wondering around, usually somewhere up near the registers.
Will never happen.
It absolutely does happen, just not for shooters who don't use their guns. My current main AR is in its second barrel, third bolt and fifth cam pin. Sock is just to pull any large metal particulate or sludge out. It works just fine and non detergent oils (or formerly detergent oils) like used motor oil and kerosene are used regularly by most gunsmiths for a variety of different things. Firearms arent going to put nearly as much pressure on said oil, or get said oil to nearly as high a temps as inside of an engine. Generally speaking, guns deal with the same carbon contaminants that are already in the oil and generate those contaminants in a lot higher concentration than an engine does in its crank case through normal use. The only practical difference anyone will ever see with used motor oil is that it doesn't have the same rust inhibbiting properties that gun oil does, which for someone who uses their gun like they do a car, doesn't matter because it gets cleaned out and replaced regularly. As far as tolerances go, depends on what part of what design as to how tight or loose they are. For guns worth training with, they're generally produced tolleranced for the carbon build up common to firearms which is way filthier than the inside of any healthy engine.
its all in the maintenance schedule.
