Can car motor oil be used as a lubricant on guns?
Yes, it can be and it will work BUT it’s certainly not the best.
Remember motor oil was designed to be used in an internal combustion engine where lubrication demands are different than a firearm. Gas engines have much different tolerances than guns do and operate at a different temperature with very little pressure. So motor oil is not the best lubricate for small arms.
Gun oils are designed to work well under the extreme conditions of a firearm. Firearm tolerances are much tighter, pressures and temperatures are much higher too. Plus they must work under harsh environmental conditions like hot and dry, hot and humid, sub-zero temperatures, windy and sandy, water, mud, dirt, salt water, etc.
If you want the best lubricant for your guns, buy a gun oil or gun lubricant. If you want something really cheap that will work, then use motor oil. Synthetic is better.