Two-Stroke and Four-Stroke Engines – What’s the Difference and What Are the Right Fuel and Oil Requirements for Each?

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There are the two main categories of marine outboard engines: two-stroke and four-stroke. Most new engines today are four-stroke, but there are still many two-stroke engines on the water.

In two stroke engines, the oil is mixed with the fuel and lubricates the engine as it passes through. It burns along with the fuel and exits via the exhaust system. Four-stroke engines are lubricated by oil that repeatedly is pumped from and returned to a sump, just as in a car or truck.

Lubricants: When it comes to the right lubricant, marine and auto oil are not interchangeable. A marine engine is engineered to be environmentally friendly, while reducing emissions and extending lifespan.

While a four-stroke marine engine performs the same function as it does in an automobile engine, marine lubricants are made to address the high level of corrosion and anti-wear protection that marine engines require.

The following types of oils can help determine the correct oil for your boat’s engine:

  • TC-W3 oils are certified for two-stroke engines
  • FC-W products are certified for four-stroke marine engines
  • FC-W Catalyst Compatible covers oils intended for use in 4-stroke engines that have an exhaust after-treatment catalyst


Avoid gasoline containing ethanol. Within the next 10 to 15 years, 15 percent ethanol (E15) is expected to become the predominant fuel in the U.S. marketplace for automotive/light duty trucks (the current standard is E10).

The Department of Energy’s National Renewable Energy Laboratory has found that E15 fuels not only damage marine engines, but also caused them to exceed EPA emission standards.

Any retailer that chooses to sell E15 fuel must post warnings at the fuel pump. Boaters should pay attention to these postings and not use E15. Using gasoline without ethanol is the safest and best course for boaters.

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