Mercury - 300 Hour Maintenance Kit FourStroke Service Package Complete Engine Care Components 8M0120838. Overview. If you run a 15-20 horsepower Mercury FourStroke, you already know what these little engines are capable of. They're the quiet, steady companions that get you out before sunrise, push you upriver when the wind turns stubborn, and carry you home after a long day on the flats. Hitting the 300-hour mark is more than just a maintenance interval-it's a milestone that shows just how much