Can I use motor oil as bar and chain oil in an emergency?

Yes, but only in a pinch. Motor oil is thinner, lacks tackifiers, and doesn’t stick to the chain at high speeds, causing more fling-off and faster chain wear. It also doesn’t handle heat well, making the bar run hotter. Plus, motor oil doesn’t biodegrade, so it’s worse for the environment than proper bar oil. Use it for one cut to get home, but don’t make it a habit.