With topics like fuel efficiency, engine oils and holeshot available for discussion, who wants to talk about gear lube? Answer: The guy who lost a gearcase because water seeped in where it didn’t belong.

Water in your outboard’s lower unit oil is a silent killer of gearcase components that usually goes undetected until it’s too late, and expensive repairs are needed. The No. 1 culprit of water ingestion is monofilament line discarded by anglers. That line then wraps around your prop shaft and damages the seals. Reusing deformed drain plug gaskets is another way water can find its way into your gear lubricant. Worn-out O-rings and seals can leak, too.

Unlike debates about whose outboard is faster, gearcase lubricants don’t stir much interest around the ramp — until your water and oil mix. To find out how lower-unit oils degrade with water contamination, we tested the lubrication characteristics of virgin gear oils. We then re-tested the same oils with a 1:6 water-to-oil ratio to simulate a leaking gearcase. This equates to about one-half to one cup of water, depending on lower unit capacity. The procedures intrigued us, the testing educated us, and the fact that some oils finished in both first and last place surprised us.



GEARCASE 101

Modern outboard lower units use spiral-bevel gears that have a curvature designed into the teeth. Unlike straight-cut gears, which impact one tooth at a time, spiral-toothed gears have three teeth in contact simultaneously — the center tooth is fully engaged with one tooth on each side partially engaged. This design distributes the forces over several teeth to increase the gear set’s load capacity. More importantly, the curved-tooth geometry results in smoother, quieter operation.

One disadvantage of spiral-bevel gears is the increased lubrication requirements due to the sliding friction along the curved teeth. This sliding action tends to wipe the lubricating film off the face of the gear teeth, and generates more friction and heat.

Spiral-bevel gears require extreme pressure (EP) additives to combat the sliding action and to hold up under the severe strains of shock loads and high rpm. These additives are activated by the elevated temperatures at the gear-tooth contact point and chemically react with the metal to form a protective barrier even when the oil film is wiped away.

It’s important not to combine different gear oils due to possible chemical reactions and altered lubricating properties. When asked about mixing lubricants, Don Wachter, a veteran engineer and consultant to the lubrication industry, replied, “As additives fight for the surface to be protected in a lubrication environment, they cannot interfere with one another. Each additive should respond at the proper temperature and pressure in its intended application.” In other words, don’t top off a low gearcase with a different lube, even if it’s from the same manufacturer — the consequences could be expensive.

Additives are formulated to be effective at different temperatures at the point of metal-to-metal contact where the heat may rise to the melting point of steel. You may have experienced the unforgettable rotten-egg smell of used lower-unit oil as it drained. This is the result of a chemical breakdown or overheating of the EP additives containing sulfur-phosphorous compounds.



GEAR LUBES

Most outboard companies recommend changing the gear oil every year or 100 hours, whichever comes first. Mercury advocates a 100-hour drain schedule in fresh water, and a 50-hour schedule in salt water. The Evinrude E-Tec outboards break tradition with their 3-year/300-hour interval when using their HPF-XR oil. Yamaha, Suzuki, Honda and Tohatsu all suggest their own gear lube or something similar to SAE 90 hypoid oil. The one exception is the Yamaha 350 hp V-8, which requires a special gear lubricant that is not to be used in other Yamaha outboards. Evinrude and Mercury recommend their own products exclusively — no substitutes.

The gear oils we chose for our tests included Yamaha Gear Lube, Amsoil 75W/80W-90 Marine Gear Lube, Mobil One 75W-90 Synthetic Gear Lubricant, Mercury/Quicksilver Premium Gear Lube, Mercury/Quicksilver High Performance Gear Lube, Evinrude HPF-Pro High Performance Gearcase Lubricant, Evinrude E-Tec HPF-XR Marine Gearcase Lubricant and WalMart Supertech 85W-90 Gear Lube.



TESTING

There are a number of tests, and a variety of devices ranging from a simple lever-operated load wheel to high-dollar computer-controlled lab machines with environmental chambers and pneumatic actuators, used to evaluate oil performance. We settled on several standardized testing methods as outlined by the American Society of Testing Materials (ASTM, astm.org) using equipment commonly found in the lubrication industry.

The testing for this article was done in cooperation with an oil consulting and engineering firm and the tests were performed following ASTM procedures and industry practices. The results posted in graphical form illustrate the changed lubrication properties with the addition of water. In one test, a few water-infused oils actually performed better, possibly from the water’s cooling effect on the sample’s temperature. Keep in mind that lab tests may not always duplicate what happens inside a hot, working gearcase.



UNDER PRESSURE

One procedure that assesses an oil’s antiwear properties is the four-ball wear test (ASTM D-4172). It consists of three stationary steel balls placed in a triangular formation, with the fourth ball pressing down and spinning against the bottom three. Visualize a pool table on which three balls are racked together and the cue ball placed on top forming a pyramid. In this test, the upper ball rotates against the others at 1200 rpm while submerged in the sample oil. After spinning for one hour with a 40-Kg (88-pound) load, the worn areas (wear scars) on the three stationary balls are measured and averaged. The test was then repeated with the water-oil mixture.



THE PRESSURE BUILDS

To make things interesting, we performed a more rigorous test using a 160-Kg (352-pound) load against the balls — four times the force of the previous test. The results were very different from our 40-Kg test. Due to the severity of the force, friction and heat generated by the load, the water-contaminated oil could not be tested consistently. Most samples sizzled and splattered like water poured into a pan of hot grease. The peak oil temperature graph shows that the lubes with better antiwear properties generated less heat on average. In both the 40-Kg and the 160-Kg load tests, the smaller the average scar, the better wear protection properties of the oil.



SQUEEZE PLAY

Our next test, the Falex Pin and V-Block test (ASTM D-3233B), evaluates an oil’s anti-wear and anti-friction properties, which is similar to the sliding action against each tooth of our spiral-bevel gears. Think of a drill press spinning a steel pin that is being squeezed tighter and tighter by two V-blocks, with the parts submerged in oil. An increased load force is applied in 250-pound increments at one-minute intervals. The test continues until the rotating pin welds itself to the V-block, thus breaking the brass shear-pin, or when the clamping force reaches about 3,000 pounds. Only the Mercury High Performance, Evinrude HPF-XR and Evinrude HPF-Pro reached the 3,000 pound threshold.



ANALYZING THE DATA

Most of the lubricants finished or tied for first place in at least one of the individual tests. This indicates that the oils are engineered for specific lubrication requirements and shows how each formula reacts in a different frictional environment.

It’s difficult to recognize a “best” lubricant, as each had its strengths and weaknesses. Each is formulated for its specific gearcase needs. It would also be unfair to evaluate an oil’s characteristics by looking at just one or two of the tests. For example, Mercury’s High-Performance Lube placed last in the 40-Kg wear test (water contaminated) yet vaulted to the top in the 160-Kg test. Yamaha and Amsoil products had both first and last place finishes as well.

It’s important to remember that you can’t go wrong by adhering to your outboard manufacturer’s lower-unit oil recommendation, as the oil is designed specifically for that use and is required for warranty coverage. If you’re into high-speed boating with engines running at full throttle, bumping the rev limiter, you may want to use a lubricant that excels in high-load and high-stress conditions. Your choices here would include Mobil 1, Mercury High-Performance Lube and either Evinrude oil, with Amsoil deserving of an honorable mention.

No one plans on having water contaminate his oil, but fishing line, worn seals or leaking gaskets are hazards boaters face each time they venture forth. You want some assurance that your gears are protected should the unthinkable occur.

After closely studying the data, we noticed one lube consistently tested well, and ranked above average in most of the tests. It also demonstrated good lubricating properties in the water dilution tests. That oil is the Evinrude HPF-XR, highly recommended for the boater who wants it all — good anti-wear characteristics, and no worries about a little water intrusion.