DIY – Mercury 2-Stroke Lower Unit Oil Change

Changing the lower unit oil on your outboard is one of the least complex things you can do as preventative maintenance. I change mine out every 6 months with Mercury-approved oil. Normally lower unit oil can be purchased at any Wal-Mart, West Marine, or marine supply store. It’s a good practice to change out your lower unit (LU) oil so debris and moisture can be removed from the gears in that part of the unit.

Over time, water and moisture may seep in, or metal particles can build up in your gears causing unusual wear and tear. Your LU oil should come out just like car engine oil, darker and less translucent than it went in; but it should not have any discoloration that doesn’t seem normal.

If a green, lime green, or brown liquid falls out of your lower unit along with the old oil, this usually means water is getting into your LU so you may have some other problems to address. A (sometimes) simple fix for discoloration in your lower unit oil is changing the seals attached to the screws which you remove to do this service.

Want to learn how to remedy a loose tiller handle on a Mercury 2-stroke? Check it out here…

Time and Difficulty

Approximately 15 minutes. Very easy!

Tools Needed

  • Flat-head screw driver
  • Oil pan/Oil bucket

Replacement Parts Needed

  • Approved lower unit oil for your outboard


There are two drain plugs on the Mercury 2-stroke outboard, and both have utilize a flat-head screw driver. They are fairly large drain plug bolts on the side of the lower unit adjacent to the propeller shaft. Place an oil pan or spent oil bucket below the outboard skeg before you begin!

1. Remove top drain plug bolt

Use your flat-head screw driver to remove the top drain plug bolt. This will allow air into the lower unit to drain properly.

2. Inspect screw seal

Once the screw is removed, take a close look at the seal (may be orange or blue) for any cracks or gashes which may be allowing contaminants such as water into the system.
 3. Remove lower drain plug
Now unscrew the lower drain plug with your flat-head screw drive but be careful, the oil will want to spill out. Keep pressure on the bolt until you have your hands situated correctly and remove the screw with the hand you’re not using to hold the screw driver.

4. Inspect drained oil

Take a good look at the spent oil in the oil pan you placed below the outboard skeg. If there is any odd looking discoloration that is brown, green, or lime green in color; you may have a leak somewhere allowing water to enter the lower unit area.

5. Fill with new oil

With both drain plug screws still removed, you may now fill the lower unit with new oil. Cut the tip off of the new oil tube and stick it into the lower drain plug’s hole and squeeze. Squeeze the tube until oil begins to leak out of the top drain plug hole. This is normal. Once it begins to overflow, stop squeezing and QUICKLY replace the tip of the oil tube with the lower drain plug screw. I simply do this as a quick swap with lightning ninja speed and give it a few screw turns by hand, then finishing it off with the screw driver.

6. Replace top drain plug screw

Replace the top drain plug screw.

All done! Go enjoy your fresh new lower unit oil.


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