Motor oil will have serious repercussions, its way more viscous so it will cause the pump to move much more slowly and it will also have much more resistance throughout the loop. Flow rate will be dramatically lower, causing temperatures to increase a lot.
On top of that, motor oil doesn't even have half the specific heat water does. in other words, it carries heat much more poorly. Once again, resulting in higher temperatures.
Motor oil isn't intended for low temperature cooling systems with weak pumps. Its meant for powerful hot systems.