How to Change an Outboard's Lower Unit Gear Oil

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Experience teaches boat owners that regular engine maintenance can help reduce those “oh no” moments we all have while boating. It’s not if something will go wrong on the water, it’s when. That’s why preventative maintenance such as monitoring and changing your lower unit gear oil is such a satisfying task.

A midseason inspection of your lower unit gear oil takes only a few minutes and costs very little compared to all the other expensive things we buy for our boats. Plus, it can provide an early warning of water intrusion, seal damage, or gear wear.

Changing Lower Unit Gear Oil Is Easy

First, even an all-thumbs mechanic like me can knock this task out in 30 minutes.

Second, an outboard’s lower unit runs on expensive and highly stressed components: gears, bearings, shafts, and seals. Inspections and oil changes can spot and fix problems before they become costly repairs.

Third, and most importantly, products and training videos can take you from start to finish with step-by-step instructions. Some manufacturers of gear oil, including Amsoil, create and share educational videos on YouTube.

Amsoil packages its gear oil in a handy container called the Easy Pack to greatly reduce the messiness of the job. That alone is worth it because, if you’re like me, changing the oil turns into an exercise in cleanup.

How to Complete the Job

Here are the steps to change your lower unit gear oil:

Buy your marine gear oil, in either an Easy Pack or gear-oil bottle and pump that threads onto the lubricant bottle.
Gather a large flat-blade screwdriver, a drain pan, absorbent rags, an optional new drain, and vent screw gaskets or washers.
Position the engine vertically.
Place the drain pan under the lower unit.
Remove the upper vent screw. This allows air to enter the gearcase and helps the oil drain smoothly.
Remove the lower drain screw and allow lubricant to drain.
Inspect the old oil. A milky or creamy appearance means water has intruded; solve that problem first before you change the oil.
Install the gear oil pump or Easy Pack.
Pump fresh oil into the lower unit.
Install the upper vent screw.
Remove the pump or Easy Pack and replace the lower drain screw.
Clean and inspect.
Run the engine on your next outing and perform a visual inspection.

How the Lower Unit Functions

The lower unit transfers all that power from the engine to your propeller through precision gears that operate under heavy loads. Clean, properly lubricated gear oil helps those gears run smoothly and prevents metal-to-metal contact and excessive wear.

Gear oil is not exposed to combustion byproducts like engine oil, but it is vulnerable to contamination from water intrusion. A damaged propeller-shaft seal, driveshaft seal, or shift-shaft seal can allow water to enter the gearcase. Once water mixes with gear oil, its lubrication quality drops dramatically and corrosion can begin inside the gearcase.

Gear oil is inexpensive compared to the cost of rebuilding a lower unit, which can easily run to several thousand dollars.

For example, an Amsoil Easy Pack costs about $25 for a one-quart pack.

Reasons an Owner Should Change Lower Unit Gear Oil

The main reason a boat owner should change the outboard’s lower unit gear oil is simple: It’s required to maintain the engine’s warranty.

Plus, not changing it can lead to a failure, and a lack of maintenance could cost the customer more money in the long run.

Engine manufacturers recommend you change your lower unit gear oil every 100 engine hours or once a season.

Consequences of Not Changing Gear Oil Regularly

So if you are forgetful or just plain lazy, not changing lower unit gear oil regularly can cause problems. The gears are under tremendous pressure, so the oil has a finite lifespan, just like engine oil. If you don’t replace the oil, it will become burnt and lose its ability to protect the bearings and gears from damage.

Another issue you may experience is fishing line in a seal. When that happens, the oil can run out and let water into the gearcase.

Boat owners may not perform this task for many reasons. They may not like the mess. Maybe they can’t properly dispose of the oil. Perhaps they are afraid to cause a bigger issue because they are not properly trained.

However, with a little research and diligence, this is a DIY task that pays dividends.

Doug Thompson

For more than 30 years, Doug Thompson has written about boats. He’s covered events such as the Catalina Water Ski Race out of Long Beach, California, cruised on personal watercraft from Key West to Key Largo for stories, told the tale of catching sailfish off Fort Lauderdale, and penned articles about new boats, marine accessories, electronics, and engines. Learn more about Doug at https://douglasmthompson.wordpress.com/.

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