Motor oil is for motors. I guess it will work short term but I really don't see the point. Are you just trying to save a few pennies on maintenance for a firearm you potentially paid 100's or even 1000's of dollars for? Why?? Just use CLP or Swiss automatenfett. Both are superior to motor oil on firearms and that's what they were designed for. I know of no first class military that issues motor oil to their troops. "Nah, you don't need anything special. Just head on over to the motor pool. Those guys will set you right up." The mere thought of it is just plain silly. They issue motor oil to the motor pool and weapon oil to the troops.
If CLP is superior to motor oil then why isn’t being used in the automotive industries? I’ve used CLP at the M60 ranges and it tends to dry up quickly and cause weapons failure. My fire team always used motor oil for better lubricatoion and sustained fire longer in competitions.