1983 Mercury 80 Hp Outboard 6333939 Gear Case Oil Changing Gearcase Oil on Your Sterndrive or Outboard Motor's Lower Unit By Tom Brunt, Concluding updated: vi/24/2019 Most sterndrive and outboard manufacturers advise you to change the gearcase oil in your lower unit every 100 hours or once a yr (whichever comes first). Clean oil is critical for protecting the heavily loaded gears in your lower unit and preventing expensive damage. Servicing the lower unit of measurement is a uncomplicated job that you tin easily practise yourself. What You'll Need to Change you Lower Unit Oil W Marine Synthetic Blend Lower Unit Gear Lube A large, directly-blade screwdriver for the lower unit drain and vent plug screws Replacement O-rings/sealing washers for lower unit drain and vent pl