Mercury - 300 Hour Maintenance Kit FourStroke Service Package Complete Engine Care Components 8M0120839. Overview. There's a certain kind of confidence you get when you know your outboard is properly cared for-the kind that lets you run a little farther, stay a little longer, and push through whatever the day brings. When you're depending on a 25-30 horsepower Mercury FourStroke, that confidence comes from routine upkeep done the right way. The Mercury 8M0120839 300 Hour Maintenance Kit delivers