Changing the lower-unit gearcase oil (also called gearcase lube) on your outboard or outdrive isn’t all that difficult. All you need is a screwdriver, a pan to catch the old oil, some paper towels or rags (lots if you’ve never done this before) and some fresh lube. Check your engine’s maintenance manual for the exact type and amount of oil you’ll need—then buy an extra quart in case you spill some. (Note: Different manufacturers recommend specific types of oil for their engines, so make sure you get the right kind.)
Ideally, the lower-unit oil should be changed at the end of the season, before winter layup. That’s mainly so you can check to see if any water has made its way into the gearcase over the season (fishing line wrapped around the prop shaft is a major culprit). If left inside the gearcase, the water can cause corrosion of internal components. Also, if a lot of water has entered the gearcase, it can freeze and expand during the winter, possibly damaging the gears, seals or housing.
Draining & Filling
Before draining the gearcase, make sure the engine is fully lowered to the vertical position to allow the used oil to fully drain from the lower unit. Next, place a towel and a wide pan under the lower unit to catch the oil, keeping in mind that the oil will start to drip off the end of the skeg as the level drops.
Next locate the upper relief-port screw and lower drain/fill-port screw on the lower unit. The lower port is where the oil drains from, while the upper port allows air to enter and the oil to drain freely. Remove the lower, drain-port screw first.
Now remove the screw to the upper port and the oil will start to drain. Be sure to set both screws aside so you can find them easily!
It’s easier to drain the lower-unit oil if the engine has been run in gear beforehand, as the heated oil will be more viscous. If the oil is cold, you may have to wait several minutes until all of it drains.
Once all of the oil has drained from the gearcase, check it for signs of water. If the oil looks cloudy, streaky or milky, water has likely entered the gearcase and you’ll have to track down the cause. The used oil should have a uniform black appearance, and should be free of any large bits of metal that could indicate gear damage. If you find any, consult a marine mechanic.
While you are at it, check to see that you have removed the washers to both the upper and lower port screws. These washers should be replaced each time the oil is changed. Fortunately, they are cheap, so buy a bunch to have on hand.
Now you’re ready to fill the gearcase with fresh oil. To make this easier, I recommend purchasing a hand pump that attaches to the quart bottle of lube and has a tube that screws into the fill port of the gearcase. This makes it much easier to fill the lower unit with fresh oil in a controlled fashion.
After attaching the pump tube to the fill port, slowly pump fresh oil into the gearcase. Do this slowly, especially in cold weather, to allow the oil to flow into different sections of the gearcase.
Once the oil begins to bubble out of the upper port, clamp off the fill tube and wait for the oil to settle into the different parts of the lower unit.
After a few minutes, remove the clamp and begin pumping in fresh lube again. Again, clamp off the fill tube once oil begins leaking out of the top port, wait for the oil to settle, then finish filling the lower unit.
Once the gearcase is completely filled with fresh lube, replace the upper port screw (don’t forget the new washer!).
Remove the fill tube and replace the lower screw. Wipe off any excess oil and you should be all set for next season.
Watch a video on how to change your lower-unit oil: