Short answer
NMMA FC-W is the National Marine Manufacturers Association certification for oils used in four-stroke, water-cooled gasoline marine engines. NMMA FC-W Catalyst Compatible, shown as FC-W (CAT), includes everything in FC-W and adds limits meant to protect exhaust catalysts. Certified oils are licensed by NMMA and carry a registration number.
Why marine four-strokes got their own standard
As large four- and six-cylinder four-stroke outboards replaced many two-stroke models, NMMA’s oil certification committee developed FC-W for the duty cycle marine engines see: long periods at high load and speed, high humidity and exposure to salt water. Automotive oil categories do not test for those conditions specifically, especially corrosion during storage in a damp environment.
FC-W follows NMMA’s earlier two-stroke programs, including TC-W3, and is administered the same way: a marketer signs a license agreement with NMMA, testing is done at a sanctioned laboratory, and approved oils are registered.
What FC-W tests
The certification manual lists identification tests plus kinematic viscosity, cold-crank and MRV viscosity, foam and aeration, shear stability, high-temperature high-shear viscosity, Noack volatility, filter plugging, a dedicated FC-W rust test, and a general performance engine test run in a Yamaha 115 hp outboard.
The oil must meet the requirements of its SAE J300 viscosity grade. The rust test is a key marine-specific element; NMMA updated its reference oils and referencing criteria in a 2023 revision of the manual.
What FC-W (CAT) adds
Four-stroke marine engines with exhaust catalysts led NMMA to create FC-W Catalyst Compatible. It includes all FC-W tests and places limits that FC-W only reports. Under the current manual, phosphorus must be between 0.06 and 0.08 percent by mass, silicon must not exceed 0.002 percent, and the oil must stay above a minimum kinematic viscosity for its grade after a 30-cycle shear stability test.
Phosphorus and silicon limits matter because those elements can reach the exhaust and affect catalyst performance over time.
Choosing an oil for your marine engine
Start with the engine owner’s manual. If it calls for FC-W, a certified FC-W or FC-W (CAT) oil fits the category; if your engine has a catalyst and the manual calls for FC-W (CAT), use an oil certified to that version. Look for the NMMA mark and registration number on the container, and match the viscosity grade the manual lists. NMMA publishes the list of registered oils.
Frequently asked questions
Can I use automotive oil in a four-stroke outboard?
Only if the engine maker allows it. Many outboard manuals call for an NMMA FC-W certified oil because automotive categories do not include the marine rust and engine tests.
Is FC-W (CAT) required for every outboard?
No. FC-W (CAT) is for engines with exhaust catalysts or where the manufacturer specifies it. Check the manual for your model.
How do I confirm an oil is FC-W certified?
Look for the NMMA FC-W or FC-W (CAT) mark with a registration number on the container, and check NMMA’s published list of registered oils.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].