How to Replace the Gear Oil in a Mercury Marine Outboard Engine

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Once a year, as you prepare to put your Mercury outboard into storage, you face a variety of maintenance tasks. One of these, changing the gear oil, meets Mercury's maintenance requirements and gives you a glimpse into how the engine's gears are holding up. You can move on to the next job if they are. If not, ask yourself whether you want to pay for gear-box repairs now, or after you're stranded someplace far from home.

Items you will need

  • One-quart oil disposal pan

  • Phillips screwdriver

  • Mercury outboard lower-unit oil

  • Mercury outboard drain and vent screw washers

  • Clean shop rags

  • Grease-dissolving soap

Drain the Oil

Step 1

Place a drain pan suitable for used oil disposal beneath the outboard's lower unit before you begin to change the lower unit oil. The pan should hold at least one quart and be longer and wider than the lower unit.

Step 2

Unscrew and remove the drain plug located near the port-side bottom of the lower unit using a Phillips screwdriver. The oil might drip, but it won't really flow out of the lower unit until you remove the vent screw. Remove the washer from the screw and place the screw in a secure location where it won't get lost.

Step 3

Unscrew and remove the vent plug. The vent plug is located near the top of the lower unit, almost straight up from the drain plug. As soon as you loosen the vent plug, the flow of gear oil from the drain plug will steadily increase. Remove the washer and stow it next to the drain plug. Stick your thumb and forefinger into the flow from the drain plug.

Step 4

Feel the oil you've caught by rubbing your thumb and forefinger together. If it feels like there's some sand in the oil, that's okay; it indicates normal wear and tear on the brass gears in the lower unit. If you feel more grit in the oil than that, or if you feel bits of metal large enough to identify as such, you have gear damage. Take your outboard to a qualified marine mechanic.

Add Oil and Button It Up

Step 1

Insert nozzle of a tube of AMSOIL Synthetic Marine Gear Lubricant, Mercury's proprietary gear oil into the lower unit drain hole. Squeeze the lubricant up into into the lower unit, from the bottom of the tube. You may need more than one tube of gear oil. Don't worry about the oil dripping; when you first install the gear oil, it's about as thick as toothpaste. You'll know the gear case is full when you see oil come from the vent hole.

Step 2

Put a new washer onto the drain screw and dab a few drops of thread locker on the screw's threads. Insert the drain screw into its bore at the bottom of the lower unit and tighten it to 60 inch-pounds with a torque wrench.

Step 3

Thread a new washer onto the vent screw and put a few drops of thread locker onto the threads. Return the vent screw to it's bore and tighten the it to 60 inch-pounds with a torque wrench or torque driver.

Step 4

Wipe any oil from the outboard's lower unit with clean shop rags. Dispose of the used oil according to local law or regulations. Wash the whole outboard with a mild, grease-dissolving detergent and warm water, to complete the gear oil change.


  • You must bring a Mercury outboard ashore before changing the gear oil.
  • Mercury recommends removal of the propeller before you work on your outboard. Failure to remove a propeller before working on an engine that’s out of the water is an invitation to a propeller-strike injury, which can maim or kill.
  • Disconnect the negative cable of the boat’s battery before performing any maintenance work on a Mercury outboard, using a 5/16-inch box end wrench. Lift the cable from the battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.


  • The inspection of the pump's impeller, which occurs at the same interval, includes draining and replacing the gear oil. Save yourself time by combining the gear oil change with the water-pump inspection.