How to Change the Lower Unit Gear Oil in a Mercury 150

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When you successfully change the gear oil in your Mercury 150 outboard's lower unit, you depend largely on the force of gravity. You should change your gear oil at least once a year, perhaps as part of a tune-up, or at 100 hours of operation, whichever occurs first. The oil drained from the gear case may tell you about leaking shaft seals or damaged gears before they become a problem.

Items you will need

  • Screwdriver

  • Gear case lubricant

  • 5/16-inch wrench

  • Adjustable wrench

Move the motor into the upright position using your trim control. Turn off the ignition, disconnect your battery and remove your propeller.

Unscrew the vent plug, located on the starboard side of the lower unit, just beneath the anticavitation plate, with a screwdriver. Discard the gasket and retain the plug.

Set a drain pan beneath the gear case and remove the drain plug with a screwdriver. As with the vent plug, discard the gasket and keep the plug. Allow the gear case to drain completely.

Inspect the drained gear case lubricant. If it has a milky appearance, water has intruded into the gear case; the propeller shaft seal may require replacement. Dip a finger into the oil and rub the oil between your finger and your thumb feeling for debris; a small amount of brass is okay, but a lot of grit may indicate a damaged shaft, gears or bearing surfaces.

Insert the tip of the gear lubricant tube in the drain plug and squeeze lubricant into the gear case until it comes out of the vent plug hole. Install a new gasket on the drain plug and thread the vent plug back into its hole. Tighten the plug with a screwdriver.

Fit a new gasket onto the drain plug. Remove the lubricant tube from the drain hole. Install the drain plug into its hole and tighten with a screwdriver.


  • When you're working on your motor, disconnect the battery cable from the negative post of the battery by removing the 5/16-inch nut from the battery post and lifting the cable from the post.
  • If you work on your outboard motor when your boat is on its trailer, or your motor is on a storage stand, remove the propeller nut with a wrench and slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before operating an outboard out of the water during maintenance or winterization is an invitation to a propeller-strike injury, which can maim or kill.