How Often Should You Change Lower Unit Oil?

Lower unit oil lubricates your lower unit (also known as a gearbox) and helps prevent wear and tear. However, just like all oil, lower unit oil wears out and needs to be replaced. After consulting with multiple marine engine mechanics and reading manufacturer owner’s manuals, here is how often you should replace your lower unit oil:

Most marine manufacturers and marine engine mechanics recommend replacing your lower unit oil once every 100 hours or once every year (whichever one comes first). This goes for both outboard and I/O motor types.

New oil is needed to protect the heavily used gears in the lower unit of your engine. Not replacing this oil as often as required can cause expensive damage or cause your lower unit to wear out quicker than usual.

Most boat mechanics offer this service with their winterizing services, and it’s fairly inexpensive, only about $50-$100. However, you can easily do this yourself for only about $10-$20, and it shouldn’t take any longer than 30 minutes of your time.

How to replace lower unit oil on an outboard engine?

Changing your lower unit oil is not very difficult to do as long as you have the right tools. These include a large flathead screwdriver, replacement O-rings, lower unit lube (check engine owner’s manual for correct kind), a lower unit cork pump, a catch pan, and rags. Click here to put all of these items in an Amazon shopping cart.

Then the process is as simple as unscrewing the lower unit drain fill plug and the upper vent plug screw. Let all the oil drain out for about an hour and then screw the lower unit cork pump into the drain fill plug and pump the new oil in.

Hear a video going step-by-step for this process:


What causes your lower unit oil to wear out?

Heat, pressure, and contamination are the three main ways lower unit oil wears out. Often times the friction between gears in the lower unit causes small pieces of metal to break off and contaminate the oil. Along with that, the moving parts create a lot of pressure and heat that degrades the quality of the oil over time.

What happens if you don’t change your lower unit oil?

To put it shortly, your lower unit will wear out and fail. Old oil will cause the gears in the lower unit to break due to friction, thus making the propulsion system fail. This may take a year or 2 to happen, but even if you do change the oil eventually, the damage is permanent and will cause your lower unit to live a shorter than expected life.