Mobil 1 10W-30 motor oil seems to work quite well as a gun oil, as well as being rather inexpensive. After a few hundred rounds through my DPMS M4gery, I had zero lubrication-related failures ((I had one due to a magazine failure. Be sure to test your gear!)) and there’s still some visible oil left on moving parts. Even the parts without visible oil are slippery and oily to the touch.
Considering the small amount of oil used in normal gun maintenance, I’m not sure that using motor oil would be a considerable cost savings compared to more expensive, gun-specific oils like CLP, Militec, etc. That said, it does seem to work well enough. It doesn’t seem to have the same cleaning properties as gun-cleaning solvents and oils like Hoppes #9, CLP, and so on, but it seems to make a great lubricant. Certainly good enough for emergency use, though reading various forums online seem to indicate that there’s lots of people who use it as their main gun oil.