I have a 18.5 B&S ran fine for about thirty minutes ran out of gas. After putting in new gas it will only run about 4 seconds blows a lot of smoke and drained a gallon of gas out of the oil reservoir. What might have caused this?

Well, if you have an oil and fuel mixture in your crankcase. It was likely caused by a small particle of debri (trash) of some sort. What happens in a lot of cases is that it only takes one microscopic particle to hold open the needle valve allowing the float to drop, and overfill the fuel bowl. Then the excess fuel flows through the intake, past the intake valve, seeps past the piston rings, and down into the crankcase (sump). That is why it is very important to always check your oil level before starting the engine. To see if it is low, or overfilled.
To remedy this, put on an inline fuel shutoff valve. Also, remove the spark plug(s), wear safety equipment, especially eye protection, tape the end of the spark plug wire with electrical tape. Hold shop towels in front of the spark plug hole, and have someone turn the engine over as if trying to start the engine. Beware, there will be either a fuel / air mixture, or a blast of fuel forced out of the spark plug hole. Drain engine oil. Refill engine with oil. Start engine, allow it to run for about five minutes, drain oil. Refill with oil and use the mower. And be sure to put on the fuel shutoff valve.