I doubt the used motor oil caused the problem. It likely already had a bad case gasket and the motor oil just finds it's way to the crancase more easily. It's just way too thin for chainsaw application.
That's possible of course. I bought it as a parts saw for the ignition that still worked. I don't know if the pump is shot or not, but I suppose it is. If it were a saw for further use, I would certainly take it apart.