Marine Engine and Gear Oil For Beginners


Boat Engines and Oil

Boat engines are unique machines requiring certain types of oil to run. That's why we've compiled this handy guide explaining some important distinctions every boat owner should know when refilling their engine. Boat Accessories Australia typically stocks the Bel-Ray range of engine and gear oils for marine applications, and their Lubricant Advisor System is a fast, convenient way of finding out which product is right for you.

Gear Oil

Most manual gearboxes rely on a sump and essentially an oil bath. Because gear oil is heavier than engine oil, it clings long enough to get pulled to the top of the gear box and coat the rest of the gears as it drains back down to the sump, much the same as rear end gears are also an oil bath in an old car.

Most marine gear oil such as the Bel-Ray Marine Mineral Gear Oil have additives to help protect against water contamination which can inhibit performance.

Engine Oil

Engine oil is lighter than gear oil but has more additives because the engine components have different tolerances, speeds and oiling mechanisms. An engine uses a pump to keep a constant flow.

Engine manufacturers typically recommend an oil change every 100 hours, and at least once a year. Even if your engine manual allows for a longer interval between oil changes, changing the oil more often will extend the life of the engine. For diesel oil, which are much harder on their lubrication, you should change the oil about every 50 hours.


Two-stroke oil is either physically mixed with the fuel or, in the case of direct fuel injection, combined with the fuel in the combustion chamber via an opening in the side of the cylinder before exiting through another cylinder port.

Initially, two-stroke engines used carburettors to control the fuel-air mixture however many of today’s two-stroke engines can use a computerised direct fuel injection system to precisely regulate the fuel-air mix to suit the operating conditions.

TC-W3 Certification

The TC-W3 Certification was created by the National Marine Manufacturers Association for two-stroke engine oil with requirements that include various bench tests for fluidity, miscibility, rust, compatibility, etc., as well as engine tests to evaluate the prevention of ring sticking and carbon buildup on pistons and other engine parts.

Once lubricants have demonstrated the necessary performance qualities needed for TC-W3 certification testing by NMMA-approved laboratories, NMMA licenses those two-stroke lubricants to be marketed to the public. TC-W3 oils are licensed around the world and are recommended for use by two-stroke engine manufacturers.

Bel-Ray Two-Stroke Oils are all TC-W3 certified. For a full list of all registered oils, check out the full list at the NMMA website here.


Four-stroke outboards use an engine very similar to an automobile’s. The air-fuel mixture flows into the combustion chamber through intake valves, and the exhaust leaves the engine via exhaust valves.

Because of these intake and exhaust valves (the valve train), a four-stroke outboard is usually heavier than a two-stroke outboard of the same horsepower. But, we see that changing: four-stroke manufacturers continue to pursue new ways to lighten the engines and extract more horsepower.

A four-stroke outboard’s lubrication system is like a car’s, complete with oil pan and filter — and the engine needs periodic oil changes to keep things running smooth.

The majority of four-stroke outboards feature sophisticated computer engine management systems and fuel injection for good performance across the power band, low emissions, and unparalleled fuel economy.

Example of Four-Stroke Engine

FC-W Certification

Like two-stroke oils, the NMMA has created a certification process for marine four-stroke oils known as FC-W certification.  The criteria for this includes bench tests for viscosity, corrosion, filter plugging, foaming and aeration. In addition, the oil must successfully pass a 100-hour general performance engine test.

Synthetic Oils

Unlike crude oil that is refined and combined with various chemical compounds for specific applications, synthetic oil is created using a base of synthesized chemicals partially derived from petroleum and combined with other chemical compounds to provide a more stable viscosity across a wider temperature range and reduce friction, especially at startup.

Additionally synthetic oil eliminates contaminants such as wax and its molecules are smaller and of more uniform size so as to produce a more even film that does a better job of keeping metal components separate.

For more information, check out the full range of Bel-Ray products here.


Category: Maintenance

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