To change lower unit oil on an outboard, remove the bottom drain plug and the top vent plug, let the old gear oil drain completely, then pump fresh marine gear lube in through the bottom hole until it flows out of the vent. Refit the vent plug first, then the drain plug, using new sealing washers. Most makers call for it once a year or at a set number of hours, so check your owner’s manual for the interval and the gear lube they specify.
What you need
- Marine gear lube of the type and quantity your engine maker specifies
- A gear lube pump or squeeze tube that threads into the drain hole
- New sealing washers for the drain and vent plugs
- A flat screwdriver or wrench for the plugs, a drain pan and rags
How to change lower unit oil step by step
- Position the motor upright (vertical, not tilted) so the gearcase drains fully and fills to the right level.
- Put the pan under the gearcase and remove the lower drain/fill plug, then the upper vent plug. Removing the vent lets air in so the oil drains.
- Let it drain completely, and inspect the oil as it comes out (see below).
- Check the drain plug magnet if it has one. A fine gray paste is normal wear; chips or chunks are not.
- Pump new gear lube in through the bottom hole until it flows out of the vent hole with no air bubbles.
- Refit the vent plug first with a new washer, then remove the pump and quickly refit the drain plug. Fitting the top plug first holds the oil in by vacuum.
- Wipe the gearcase clean and check for drips after the next run.
What the old oil tells you
| What you see | What it means |
|---|---|
| Clean, honey or dark colored oil | Normal |
| Milky or creamy oil | Water is getting in, usually past the propeller shaft or shift shaft seals; have the seals replaced before refilling and running the motor |
| Water drains out first | Serious water intrusion; do not run the motor until the gearcase is repaired |
| Metal chips, chunks or a burnt smell | Gear or bearing damage; have the gearcase inspected |
Water in the gearcase is especially harmful in freezing weather, when it can expand and crack the housing, which is why the oil change is part of most outboard winterizing routines.
How often to change it
Many outboard makers call for a gear oil change at least once a year or at a set number of operating hours, whichever comes first, and some call for an earlier first change on a new motor. Follow the schedule in your manual, and check the oil sooner if you hit something with the propeller, since a bent prop shaft can damage the seals. More routine checks are in our outboard motor maintenance guide.
Which gear lube?
Use a marine gear lube that meets your engine maker’s specification. Mercury, Suzuki, Tohatsu and Yamaha each publish the gear lube type and gearcase capacity for their models in the owner’s manual; capacity varies a lot with motor size. Shopping for a new motor instead? Browse our outboard motors by horsepower, or the Suzuki, Tohatsu and Mercury ranges.
More guides: first service after break-in · milky lower unit oil · how to flush an outboard · outboard won’t go into gear · how to change outboard engine oil.
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