Mercury - 300-Hour Maintenance Kit 40-60HP FourStroke EFI Command Thrust OEM Service Components 8M0090559. Overview. Most boaters can tell you exactly where they were the first time their engine reminded them that maintenance matters. Maybe it was a sluggish start on a warm morning or a rough idle as you eased into the no-wake zone. Moments like that make you appreciate the value of doing things right - and doing them on time. The Mercury 8M0090559 300-Hour Maintenance Kit gives you everything y