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.
Read more on how to change your lower unit gearcase oil.