Consider the differences in how you use your marine engine versus your car engine. Start with the amount of use: Most cars run every day, while most recreational-fishing boats sit idle for days, frequently in damp, salty environments. When under way, your marine engine operates under constant load, even while trolling; when cruising, it runs at a relatively high number of revolutions per minute. Autos, on the other hand, can lope along in overdrive at lower revs, and often have the luxury of coasting. No such luck with a boat.
These key differences mean that the marine drivetrain in your fishing boat needs specialized lubricating fluids. Let’s take a look at factors to keep in mind when selecting oils for your boat engine.
Whenever asked about the best marine oils, I have a simple answer: Buy the same brand as your engine. Builders such as Cummins (marine.cummins.com), Detroit Diesel (detroitdieselmarine.com), Evinrude (evinrude.com), MerCruiser (mercurymarine.com), Suzuki (suzukimarine.com), Tohatsu (tohatsu.com), Volvo Penta (volvopenta.com), Yamaha (yamahaoutboards.com) and Yanmar (yanmarmarine.com) have vested interests in keeping their engines running strong, particularly during the factory-warranty period, which might stretch to six years. So it makes sense that the factories formulate oils to keep their power trains in the best possible condition. Availability of factory brands is often limited to authorized dealers, and these products usually carry a premium price. Call it cheap insurance.
In addition, a few marine engines need specialized oils from the factory. For example, upon request, some Evinrude E-Tec outboards can be programmed by an authorized dealer to minimize the amount of two-stroke oil they consume. Once reprogrammed, however, such an outboard requires Evinrude’s XD100 synthetic two-stroke oil in order to operate reliably.
Also, when it comes to a four-stroke gasoline or diesel engine, the builder will specify a grade of oil, such as 10W-40, though it might list a range of grades to use, depending on the season or climate. A builder will also specify the grade of gear oil to use in a lower unit or transmission, such as 90W or 80W-90. Check your owner’s manual to make sure you’re using the recommended grades, or you might invalidate your engine warranty.
Whether shopping for factory brands or not, use only marine-engine oils that have passed the National Marine Manufacturers Association (nmma.org) certification test, indicated on the label as TC-W3 for two-stroke oil and FC-W for four-stroke oil. This means the oil contains additives to deal with the unique combination of harsh factors facing marine engines, such as long periods of inactivity, moisture, corrosion, carbon deposits, greater loads and higher operating speeds. It also means that samples of the oil — be it engine-branded or an independent brand such as Mystik Lubricants (mystiklubes.com), Pennzoil Marine (pennzoil.com) or Sierra Marine (teleflexmarine.com) — have been tested for effectiveness in the demanding marine environment.
The NMMA does not have a certification program for gear oils, but it’s important to buy brands formulated for marine use, as lower units and gear boxes face many of the same challenges as marine engines. Once again, you can’t go wrong using the same brand as your drivetrain, but brands such as Mystik, Pennzoil and Sierra also offer marine-specific gear oils. Avoid using any ol’ gear oil you pick up at an auto-parts store.