Joe Caption mentioned motor oil slinging off. It sure does! The oil coats everything in front of, and under, the bar.
But, I've used it in a pinch. The oil sump empties a lot faster, so keep watch on it. Bar oil has tackifiers in it to help it cling.
A small bit of HyperLube added to the motor oil will increase tackiness quite a bit.
Sometimes one can find what is called "Way Oil" for less money than bar oil. It's for the ways of a machine tool, such as a lathe or vise.
Way oil has tenacious tackifiers in it and clings to bars, chains and sprocekts very well even when the parts get hot from working. (Discovered this by mistake when I ran out of bar oil and way oil was the only sticky oil I had.)
Paul
PS: Don't use used motor oil. It may have evil byproducts of combustion in it and will have at least some abrasives.