Two-stroke 15-horsepower Mercury outboard motors require a 50:1 fuel-to-oil mixture. However, not all 15-horsepower Mercury outboards are two-stroke engines; many newer models are four-stroke designs. Four-stroke engines have a lubrication system similar to a car, with a separate oil reserve, and don't use pre-mixed fuels.
It is important to use the correct fuel for each specific engine type. Two-stroke engines do not have a valve train with valves, camshafts, and chains or belts. The oil in the fuel mixture lubricates the engine. A four-stroke engine has an oil pan and oil filter and needs oil changes periodically for smooth operation.