The Lowdown on Lower Unit Lubrication - 07/01/2009

Conversations about lower unit lubrication don't typically generate much interest around the launch ramp. That is until someone gets water in it and loses a gear case. It happens more than you may realize because all of the power and torque of your engine turning at thousands of rpms is concentrated on two small rotating metal surfaces. We’ve seen more than a few boaters spring for a whole new engine because of a blown gear case. Here's what you need to know about outboard lower units and why choosing the right lubricant is so critical.

Outboard Lower UnitRoyal Purple Max-Gear
Outboard lower units develop tremendous heat in their spiral-bevel gears and need a high “film strength” gear oil to lubricate metal-to-metal contact and prevent damaging wear. ASTM Timken load tests show that Royal Purple Max-Gear has a far stronger “film strength” than both synthetic and mineral oils. 

The Design Challenge

Modern outboard lower units use spiral-bevel gears designed with a curvature in their teeth. This design distributes the load over several teeth to increase the gear set's load capacity providing smoother, quieter operation. The downside of this design is the increased lubrication demand due to the sliding along the curved teeth. The sliding action increases friction and heat. These extremely demanding conditions require specific lubrication.

These spiral-bevel gears illustrate the area of contact by tremendous torque creating heat and wear.
The Lubrication Challenge

Spiral-bevel gears require extreme pressure additives (known in the industry simply as EP additives) to handle the strain of increased shock load and high RPMs. It's important that a lubricant has adequate film strength to prevent the metal to metal contact that causes heat and wear. Film strength can be measured using American Society for Testing Materials (ASTM) D-2782 Timken Load test. The higher the film strength, the less metal to metal contact. Performance lubricant manufacturer Royal Purple has gained a reputation for developing lubricants that possess very high film strength as exhibited in the chart comparing the results of the leading synthetic gear oil and conventional (mineral oil based) gear oil with Royal Purple's 'Max-Gear' gear oil.

The American Society for Testing Materials (ASTM) D-2782 Timken Load test shows that Royal Purple Max-Gear oil has far higher “film strength” than conventional oils.

It's important that different gear oils not be combined. If you elect to switch gear oils you should make sure all of the old gear oil is completely drained out before adding the new gear oil. Each gear oil brand formula is unique; mixing two brands will result in a lubricant that is inferior to either of the two that you started with.

The Frequency

Most outboard manufacturers recommend changing gear oil annually or every 100 hours (whichever comes first). Some recommend shorter intervals when in salt water. At least one company, Evinrude, allows up to a 300 hour/3 year drain when using their particular HPF-XR oil. Regardless of the change interval, any accidental introduction of water into the system demands immediate attention.


Fishing Wire, Caught
Fishing line around a prop.

Water - The Unseen Enemy

Water in an outboard's lower unit often goes undetected until it's too late and expensive repairs are needed. One of the most common causes, particularly among anglers, is discarded monofilament line which gets wrapped around the prop shaft and damages seals.

When water and oil are violently mixed, the emulsified mixture dramatically reduces the gear oil's ability to do its job. A lubricant's ability to rapidly separate from water indicates how well it will continue to lubricate should water be introduced to the system. (Regardless of how well and quickly a lubricant separates from water, it should be changed immediately if water enters the system.) The chart below indicates how well the Royal Purple gear oil separated from water when compared to the leading synthetic and conventional gear oils.


Gear Oil
The faster gear oil can separate from water, the faster it can start protecting your gears. This test shows that after only 20 minutes Royal Purple Max Gear was completely separated from water and after 60 minutes conventional oils were still partially emulsified.

No one would ever want or plan on having water contamination in their lower unit. Spending a few extra dollars for the extra protection from a high quality gear oil is likely worth the peace of mind.

Royal Purple has over 18,000 dealers in the U.S. To find the closest dealer to you…

Max Gear
Royal Purple makes a full line of lubricating products that outperform conventional synthetic and mineral oils.

To find out more about its full line of products go to