The gears in an outboard's lower unit run in a bath of gear lube, and it is the only thing protecting them. Changing it takes a pump, a pair of new gaskets and a drain pan, and the old oil tells you whether the seals are letting water in.
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Fill from the bottom hole until oil shows at the top vent.
Find the drain and vent screws
The drain screw is at the very bottom of the gearcase. The vent screw is higher up on the side. Honda calls them the oil drain plug and the oil level plug. Oil goes in through the bottom hole and comes out of the top one when the gearcase is full.
Use a screwdriver that fits the slot well. Honda calls for a blade at least 10 mm by 1.5 mm, a #3 Phillips bit rather than a #2 on Phillips screws, and an impact driver if a screw won't break loose. On some Mercury gearcases the propeller has to come off to reach them.
Pump oil in at the drain screw until it seeps from the vent.
How to change lower unit oil
Stop the engine and stand the outboard upright
Work with the engine off and the outboard vertical. Don't run it just before you start: Yamaha notes that churning traps fine air bubbles in the oil, which can look like the milky color of water.
Remove the drain screw, then the vent screw
Put the pan under the gearcase. Yamaha and Honda both take out the bottom drain screw first, then the upper vent screw; follow your own manual if it says otherwise. Let all the oil drain out. Honda tilts its larger motors up slightly while draining so the drain hole is at the lowest point.
Check the magnet on the drain screw
Wipe the magnet on a clean shop towel. Fine metal dust is normal. Metal chips, on the screw or in the pan, mean the gearcase should be checked by a dealer.
Look at the old oil
Milky or creamy oil has water in it, usually through a leaking seal. Oil that is black and smells burned is also a reason to have the lower unit checked. The table below covers what each one means.
Replace the gaskets
Put a new sealing washer or O-ring on each screw every time they come out; Yamaha says they are not reusable and are a main defense against water getting in. If you can't find the old one, it may still be stuck to the gearcase. Pick it off.
Pump new oil in from the bottom
With the outboard upright, thread the pump fitting into the drain hole and pump slowly until oil seeps out of the vent hole. Yamaha then waits five minutes and pumps slowly again until it seeps out a second time, which lets trapped air out.
Vent screw in first, then the drain screw
Install the vent screw with its new gasket. Then remove the pump and quickly put in the drain screw with its new gasket. Losing a little oil while you swap them is normal. Tighten both to the torque in your owner's manual. It is a small number: Honda's is 2.6 to 4.8 lbf·ft across its range.
Clean up and recycle the old oil
Wipe off any oil on the outboard. Pour the old oil into a sealed container and take it to a recycling center. Don't put it in the trash, on the ground or down a drain.
What the old oil tells you
What you see
What it means
What to do
Clean oil, fine metal dust on the magnet
Normal wear
Refill and carry on
Milky, creamy or white oil
Water in the gearcase, usually a leaking seal
Have a dealer pressure test the gearcase and replace seals if needed
Water runs out first
Water in the gearcase
Have the lower unit checked by a dealer
Black oil that smells burned
A warning sign to both Honda and Yamaha
Have the lower unit checked by a dealer
Metal chips on the magnet or in the pan
Possible internal damage
Have a dealer check the gearcase before you run it again
How often to change lower unit oil
Yamaha's schedule is after the first 20 hours on a new outboard, then every 100 hours. Mercury says every 100 hours or once a season. Your owner's manual has the interval for your model. If you only do it once a year, do it before storage, when the winterizing checklist calls for it anyway.
Between changes, pull the propeller now and then and check for fishing line wrapped around the shaft. Both Yamaha and Mercury warn that line can cut into the prop shaft seal and let water into the gearcase.
Which gear oil to use
Use the gear lube your owner's manual names. Yamaha says to avoid automotive and tractor gear oils, because a marine lube has to keep lubricating even with some water in it. Honda specifies its own gear case oil or a marine SAE 90 hypoid gear oil rated API GL-4 or GL-5. Mercury recommends a marine-specific lube such as its High Performance Gear Lube.
How much oil does a lower unit hold?
It depends on the model, so check the owner's manual. Honda's own charts alone run from 1.7 ounces to 49.6 ounces across its models. Filling from the bottom until oil comes out of the vent tells you when it is full, so buy a little more than the manual's figure.
Common questions
Water has got into the gearcase, usually past a seal. Have a dealer pressure test it. Don't confuse it with the fine bubbles you get if you check the oil right after running the engine.
Yamaha says after the first 20 hours on a new outboard and every 100 hours after that. Mercury says every 100 hours or once a season. Check your owner's manual.
Yamaha and Honda both say the bottom drain screw, then the upper vent screw, and the reverse when refilling, vent screw in first and drain screw last. Check your own manual, since procedures differ between engines.
No. Yamaha says the crush washers are not reusable and replace them every time the screws come out.
Fine metal dust is normal. Chips or larger pieces mean the gearcase should be checked by a dealer.
No. Yamaha says to avoid automotive and tractor gear oils. Use a marine gear lube of the type your manual specifies.
Specifications come from manufacturers' published documents and can change. Check your engine or boat maker's current manual before you change a setting or fit a part.