Outboard Lower Unit Oil Changes: The Basics

You know you have to change your outboard’s engine oil, but one maintenance task that's often overlooked is changing the lower unit oil (gearcase lube).

Lower unit oil change basics

Here at Boats.net, we’ve published many videos and articles about changing lower unit oil for Mercury and Yamaha outboards. For this article, we’re breaking down just the basics of changing outboard lower unit oil.

Tools and Supplies - Outboard Lower Unit Oil Changes

Buy outboard lower unit oil

Buy outboard gearcase lube pumps

NOTE: You’ll also want to have replacement washers and/or O-rings on hand for the drain and vent plugs in case they’re worn or go missing.

What is Lower Unit Oil?

Simply put, gearcase lube is oil formulated at different viscosities and with different additives than engine oil to cool, lubricate and protect the moving parts of an outboard’s lower unit. Without lower unit oil changes, heavily loaded gears wear faster and eventually break. It’s every bit as important as changing the engine oil, but the oils for both tasks are not interchangeable. 

How Often Should You Change Outboard Gearcase Oil?

Once a year is the recommended bare minimum for changing lower unit oil, but if you use your boat often, you should change it more often. Changing the lower unit oil after every 100 hours of use is highly recommended. Always follow the manufacturer’s recommendations when choosing gearcase lube. 

Changing Outboard Lower Unit Oil: Basic Steps

  • Put the outboard in a vertical, upright position.
  • Have a drain pan ready to catch the oil below the lower drain plug.
  • Remove the upper oil vent plug from the gearcase.
  • Remove the lower drain plug with a flathead screwdriver and let the gearcase oil drain out completely.

Outboard lower unit oil change drain screw

  • Connect a gear lube pump to a bottle of gearcase oil and the hose end of the pump to the lower drain plug.
  • Pump gearcase oil into the fill hole until it starts to overflow from the vent hole at the top.
  • Reinstall the oil vent screw with its washer attached.
  • Remove the fill pump hose from the oil drain plug hole, then reinstall the plug and its washer.

It’s as simple as that. When changing the lower unit oil, keep the following things in mind:

  • Have rags ready to clean up any spills.
  • Be careful not to let the oil drain plug or its washer fall into the drain pan.
  • Inspect the washers for both the drain and vent plug screws for wear and replace them if necessary.

Outboard lower unit oil chnage washer inspection

  • If the drain screw is stuck because of corrosion, use a penetrating lube on it and let it sit overnight to loosen the screw.
  • Gear lube pumps are often manufacturer-specific, and come with an adapter to fit an outboard’s specific make and model.

Outboard gearcase lube change

  • You can’t go wrong with the manufacturer’s recommendations on what gearcase lube to use, but just make sure whatever brand/type of lower unit oil you use is marine grade. 
  • When changing the gearcase oil, take note of the old oil’s appearance as you’re draining it out. Lower unit oil with a milky appearance or with metal shavings in it are signs of serious engine problems, so keep an eye out for those. 

Outboard lower unit milky oil

  • Running an outboard engine without completely filling the lower unit with oil to the proper level will damage its moving parts.
  • Dispose of the old lower unit oil at a local oil recycling center.


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