subject: How to Replace Outboard Gearbox Oil [print this page] It is very simple and easy to change the gear oil or gear lube in the lower unit or gearbox of your outboard motor. The most important thing to remember is to fill the gearbox or lower unit from the bottom drain hole.
A lot of people struggle at first to accept this but it is most important. If you fill the gearbox from the top hole, unless you are VERY LUCKY you will get an air lock in the gearbox, this will be a big bubble of air that takes up the space required for the gear oil to fill. The oil will start over flowing out of the top hole and you assume that the gearbox if full and stop filling it. A gearbox without the correct amount of gear lube in it will fail very quickly.
With the outboard motor still tilted up, loosen both drain screws. It is easier to loosen the bottom screw with the outboard tilted up. Tilt the outboard down as far as you can. If the prop shaft can't be horizontal, or angling down, you will need to drop the front of the trailer or jack the back of the trailer up so you can lower the motor.
Remove outboard drain screws and drain oil into a container. There will be two screws, one at the bottom of the gearbox on the side and one at the top of the gearbox, usually on the same side as the bottom screw. Inspect old gear oil for signs of metal or water contamination. Most outboard motors will have small magnets on the lower unit drain screws. Inspect these magnets for signs of metal. If it doesn't have magnets, look at the oil in the sun, metal filings will be shiny in the sun. Water will either drain out first before the oil, or it will have mixed with the oil and made the oil go a white milky colour.
