May I use 5-30w motor oil even through the owners manual states use only 30w motor oil in a OMC 1986 marine engine.
Yes you can you can. In fact it will allow the engine to turn over easier during a cold start.
But, and I have to add this, since straight 30 weight oil is still widely available,and less expensive than multi-wieght oils, why not just use what's recommended? You can put the 5W30 in your truck or car, so it wont go to waste.