What type of lower unit fluid should I use?

Choosing the Right Lower Unit Fluid: A Guide

Proper lower unit fluid is crucial for the longevity and performance of your outboard motor. While some may suggest different fluids based on horsepower, the best practice is to always use a full synthetic lower unit fluid.

Here’s why:

• Superior Protection: Synthetics offer superior resistance to water contamination, extreme temperatures, and high pressures encountered in modern outboard motors. This translates to better protection against wear and tear, corrosion, and foaming.
• Enhanced Performance: Full synthetics provide smoother gear operation, improved fuel efficiency, and reduced noise levels.
• Extended Service Life: Their robust formulation allows for longer service intervals between fluid changes, saving you time and money.

Don’t be misled by the size of your motor. Even smaller outboards with lower horsepower operate under significant stress. Their compact lower units can be particularly susceptible to fluid breakdown, making the use of high-quality synthetics even more critical.

What to Look For:

• GL-5 Rating: Ensure the fluid meets the GL-5 specification. This indicates it has the necessary extreme-pressure (EP) additives to protect gears under heavy loads.
• Marine-Specific Formulation: Choose a fluid specifically designed for marine applications. These fluids are formulated to resist water intrusion and provide optimal performance in saltwater environments.

Joe’s Summary: It’s best to go with a full synthetic lower unit fluid regardless of horsepower. The smaller lower units have a small capacity and can break fluid down just as easily as the larger units. High horsepower units definitely need the sheer strength of the best synthetics. Any fluid meeting a GL5 rating should work just fine.

<See How to fill your lower unit here>

<Learn about the GL5 Oil Rating>

We have new and used parts for lower units.  We do recommend new gears, seals, and bearings. But we can quote used shafts, carriers, shifters, etc.

Depending on our workload we can usually rebuild your lower unit in 2 to 4 days. Shipping time to get it here and ship it back is a bigger consideration depending on how far you are from northern Michigan.

We have approximately 150 lower units rebuilt in stock. Advanced Marine also has over 400 core lower units to help us build even more.

We buy our gears from an American owned company located in Wisconsin. Last year we had the opportunity to take a tour of the factory and see the 1.2 million dollar gear cutting machine in action. The technicians running the machines were happy to talk to us about their jobs. We spend about $50,000 annually just on gear sets with this company.

Gears are not as simple as they appear to be. The 1st step to creating a quality gear set is the alloy of the metal that goes into them. The 2nd part of a gear set is the precise cut of the helical gears. The 3rd critical part is the heat treating process. We have found that gear sets that were sourced overseas do not have a reliable consistency in the quality. Therefore we choose American made gears that have remained consistent in quality over the 20 years we have been using them.

Most lower units are filled from the bottom. The one exception is a Cobra lower unit, which we will discuss later. Fill from the bottom hole until lower unit oil flows from the the top hole. Put the top screw in. Then put the bottom screw in. After initial run top off thru the upper hole.

We ship the same day you order if it’s early in the day, or the next day.

All parts and units will ship from Northern Michigan.

If you are sending your lower unit to us to be rebuilt it will typically be in the shop 2-3 days before being shipped back.