January 2025 Column Marine Engine Oil…it’s different from what you put in your car. By Capt. John N.Raguso
Most outboard marine engines sitting on the stern of a variety of craft at your local marina are four-stroke gasoline fueled motors. Sure there are still a few two-strokes to be seen from the early 2000s, along with the occasion diesel or propane flavors, but if you look up and down the docks, four-stroke gasoline outboards are the power of choice. A question I get asked a lot as a seminar speaker, writer and charter boat captain is,“is it okay to use automotive engine oil in my four-stroke outboard, inboard or stern drive?”My simple one-word answer is “no”, that’s a really bad idea. Now let’s take a deeper dive into myriad reasons for this inflexible, absolute response. DIFFERENT STROKES Automotive and marine four-stroke engines are designed to run quite differently. In order to meet emissions and fuel economy standards, automotive engines are calibrated to oper- ate within a certain RPM range and seldom see any heavy loads. They are typically run- ning at low revs (1500-2000 rpm), even at 55-to-65 mph highway speeds, at a controlled temperature that ensures consistent operation and fuel usage. In contrast, marine engines consistently generate higher RPM (3500-to-5000 rpm or more) and loads, as if the engine was trying to push up a never-ending hill mired in first gear. Marine engine oil has to resist shear, moisture and corrosion more than automotive engine oil and has to have extremely strong film strength to protect engine parts under constant stress and load. Finally, marine engines often go past their 100-hour or once-a-season oil change window, so anti-oxidants are critical to extending oil lifespan. The function of engine oil is to reduce friction and wear; carry away engine heat by being circulated and cooled in a closed loop system; to minimize rust and corrosion by preventing air or moisture from contacting bare metal; to remove contaminants by carrying them to a filtration system and/or settling them in a reservoir; and to seal and lubricate flexing piston rings during the four-stroke combustion process.
According to Mercury Marine engineers, engine attitude in out- board applications is much different than in an automotive appli- cation, with the engine most often configured vertically. The job of an oil in a vertically-oriented engine is even more challenging because it has to flow up and down while providing consistent protection throughout the engine. Lubricating cylinders and bearings at the top of the engine can be more difficult and lubricating individual cylinders, crankshaft and rod bearing journals can also be challenging.
Because marine engine oils have to protect against so many marine-specific things (corrosion, oxidation, shear, moisture), additives are much more important in a marine oil blend than in an automotive blend. Accordingly, marine blends typically contain 20-to-35% additives, while automotive blends contain about 10- to-20% additives. Mercury’s Quicksilver marine engine oils contain significant amounts of anti-wear additives that help protect marine engines from friction that can result from excessive loads and the challenges brought on by their configuration. In short, automotive engine oils are blended for mileage, emissions and efficiency and are not suitable for use in marine engines. Marine Engine Oils are tested and proven to protect against rust and corrosion, offer increased wear protection and have optimum viscosity retention. They have also been extensively lab-tested and proven to protect marine engines better than leading automotive engine oils. Using automotive oils in your marine engine can also void its warranty…yikes!
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