How to Change the Oil in a Mercruiser Lower Unit

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Every 100 operating hours or once yearly, you are supposed to change the gear lube in the lower unit of your Mecruiser sterndrive. If you do not adhere to this schedule, when you finally do get around to changing the gear lube, the oil will be filled with chunks of brass that were formerly your gears. While the Mercruiser shop manual isn't quite that straightforward about lower unit oil changes, you do get the sense that very bad things will happen unless you change the oil. The oil change itself is a straightforward process.

Items you will need

  • Oil absorbent pads

  • 25 oz. oil drainage basin

  • Screwdriver

  • Drain screw washers

  • Gear lube pump

  • Gear lubricant

  • Mild soap

  • Water

Remove the propeller and disconnect the negative cable from your battery. Place oil absorbent pads under and around the lower unit and set a drainage basin -- one capable of holding 25 oz. of used oil -- underneath the stern drive's lower unit. Use the trim control to lower the stern drive until the propeller shaft housing is parallel to the floor. Reposition the drainage basin as necessary to catch the oil -- the Bravo One drive drains at the aft end of the propeller shaft housing. Other Mercruiser stern drives drain at the center of the propeller housing.

Locate the gear lube monitor on the top, front, port side of the motor. Unsnap the strap across its face and remove it from the bracket. Remove the cap and dump the contents of the monitor into the oil change basin. Return the gear lube monitor to the bracket. Do not replace the cap, but inspect the rubber gasket inside the cap, and replace the gasket, if necessary.

Locate and remove the fill/drain screw with a screwdriver. Note that the fill/drain screw for Bravo One stern drives is located inside the aft end of the propeller shaft housing. The fill/drain screw for all other Mercruiser stern drives is located on the starboard side of the bottom of the propeller shaft housing.

Locate and remove the oil vent screw on the starboard side of the upper part of the stern drive -- it's the only screw visible in that area. Allow the 22.5 oz. of lower unit oil to drain. Put new washers on both the fill/drain screw and the vent screw while the oil drains. Swill your fingers in the oil and rub them together, taking note of any bits of metal. Note the color of the oil -- milky oil means water in the oil. That means a trip to the dealer.

Connect the intake hose of the gear lube pump to the lubricant tube and feed the pump's discharge hose into the oil fill/drain bore. Pump lubricant into the lower unit until a stream of lubricant -- without air bubbles -- flows from the vent hole.

Thread the vent screw back into its bore and tighten it securely. Return to pumping oil into the stern drive. Continue pumping until gear lubricant fills the sight glass of the gear lube monitor to the center of the operating range marked on the glass. Replace the cap on the gear lube monitor and tighten snugly.

Pull the gear lube pump's discharge hose from the fill/drain bore and place the fill/drain screw in the bore before the lubricant begins to run out. Tighten the fill/drain screw securely. Wash the motor with mild soap and water.


  • Disconnect the negative cable of your battery before performing any maintenance work on your outboard motor, to prevent electrical shock or accidental starting. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from your 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.
  • 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 long-term storage is an invitation to a propeller-strike injury, which can maim or kill.


  • After you first use the motor after changing the lower unit oil, let the motor stand until it's cold. Check the oil level in the gear lube monitor. You do this when the engine's cold because, during use and while the motor is hot, the gear lube level will rise and fall.