Specially engineered for marine applications. Durable formulation protects in severe conditions, from all-day trolling to high-rpm operation. Heavily fortified with advanced additives designed to prevent deposits, rust, and corrosion. Use in gasoline-fueled four-stroke inboards, outboards, inboard/outboards and personal watercraft, including those made by Honda,* Mercury,* Yamaha,* Johnson/Evinrude,* Bombardier/BRP,* Suzuki,* Nissan,* Tohatsu,* OMC,* Volvo-Penta,* Mercruiser,* Chevrolet* and Ford.*
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Product data sheets
View, download, or save a copy of the Product Data Sheet or Safety Data Sheet for AMSOIL 10W-40 100% Synthetic Marine Engine Oil.
AMSOIL 10W-40 100% Synthetic Marine Engine Oil (WCF) is a premium synthetic four-stroke marine formulation built for gasoline-fueled inboards, outboards, sterndrives, supercharged watercraft engines, and personal watercraft. The 10W-40 grade carries an NMMA FC-W (CAT) Catalyst Compatible registration (FC-00850T) and meets API SM and JASO MA, making it a strong match for warm-water operation and higher-output four-stroke engines that benefit from a heavier high-temperature, high-shear viscosity.
Engine wear protection at high rpm
Naturally strong synthetic base oils resist viscosity loss from mechanical shear, so the 10W-40 grade holds its working grade through hard pulls, long trolling sessions, and sustained wide-open-throttle runs. A tough lubricating film protects rings, bearings, cams, and valve train under the high-heat, high-rpm conditions that thin and break down lesser oils, while robust anti-wear additives reduce metal-to-metal contact and guard against compression loss. Four-Ball Wear testing (ASTM D4172) records a 0.40 mm wear scar for the 10W-40 grade, with a high-temperature/high-shear viscosity of 4.2 cP (ASTM D5481).
Rust and corrosion control during storage
Good rust protection is not natural to motor oil; it comes only by design. The formula is fortified with potent rust and corrosion inhibitors that protect cylinder walls, valve train, and bearings during use and through extended periods of inactivity. That matters for outboards, sterndrives, and personal watercraft that spend months in seasonal storage, sit on a lift between weekends, or run in humid, salt-laden environments where condensation forms inside the crankcase.
NMMA FC-W Catalyst Compatible certification
The 10W-40 grade is certified by the National Marine Manufacturers Association for use in four-stroke outboards and carries FC-W (CAT) Registration Number FC-00850T, the catalyst-compatible specification that is backward-compatible with the original NMMA FC-W standard. Engines fitted with catalytic converters and emissions hardware see additive chemistry tuned to keep aftertreatment systems clean, while engines that simply call for FC-W see the same approved oil without compromise. The sister 10W-30 grade (WCT) carries FC-W (CAT) Registration Number FC-00833T for engines that specify the lighter viscosity.
Heat and oxidation stability
Four-stroke marine duty cycles run hot. Long idle and trolling stretches followed by sustained high-rpm runs put more heat into the oil than typical automotive service. The synthetic chemistry resists thermal breakdown and oxidation, with a flash point of 240 C (464 F) by ASTM D92 and a viscosity index of 155 (ASTM D2270) that holds the oil thicker at operating temperature and thinner at startup than a conventional 10W-40. NOACK volatility of 7.5 percent weight loss (ASTM D5800) limits evaporative oil consumption between scheduled changes.
Cold-start flow and pour point
A pour point of -38 C (-36 F) by ASTM D97 keeps the oil fluid through cold-morning starts at the launch ramp and through off-season starts during winterization or pre-season checkout. The 10W cold-cranking grade circulates quickly on startup, reducing dry-start wear.
Detergency, dispersancy, and warranty coverage
A robust detergent and dispersant package suspends combustion soot, condensation, and partial-fuel byproducts so they pass into the oil filter rather than depositing on rings or valve train. A Total Base Number of 7.9 provides reserve alkalinity to neutralize acidic combustion byproducts through the service interval. The 10W-40 grade is Warranty Secure and keeps a new factory warranty intact when used in place of the manufacturer-branded oil; it is also backed by the AMSOIL Limited Warranty.
Recommended applications and service intervals
AMSOIL 10W-40 100% Synthetic Marine Engine Oil is approved for gasoline-fueled four-stroke marine engines, including inboards, outboards, sterndrives (I/O), supercharged watercraft engines, and personal watercraft from Honda, Mercury, Mercruiser, Yamaha, Johnson/Evinrude, Bombardier/BRP, Suzuki, Nissan, Tohatsu, OMC, Volvo-Penta, Chevrolet, and Ford. Specification coverage is as follows:
Four-stroke outboard motors and personal watercraft: change oil at the engine or watercraft manufacturer-recommended interval due to the potential for fuel dilution or water contamination in marine duty.
Inboard and sterndrive (I/O) applications: change oil at two times the engine manufacturer-recommended interval or one year, whichever comes first.
NMMA FC-W Catalyst Compatible, API SM, JASO MA. FC-W (CAT) Registration Number FC-00850T for the 10W-40 grade.
Not recommended for two-stroke outboards (use a TC-W3 or equivalent two-stroke marine oil) and not intended for diesel marine engines.
The 10W-40 grade is compatible with other synthetic and conventional motor oils, but mixing shortens the life expectancy of the AMSOIL oil and reduces performance benefits, and extended drain intervals are not supported when oils have been mixed. Aftermarket oil additives are not recommended for use with this formula. Pair the oil change with a fresh, application-correct marine oil filter and follow the engine manufacturer service procedure for fuel filter and water-separator service at the same interval.
NOACK Volatility, percent weight loss (ASTM D5800)
7.5
Total Base Number
7.9
Order AMSOIL 10W-40 Synthetic Marine Engine Oil
AMSOIL 10W-40 100% Synthetic Marine Engine Oil is available in quart bottle (WCFQT-EA), case of 12 quarts (WCFQT-CA), gallon jug (WCF1G-EA), and 55 gallon drum (WCF55-EA) sizes. Using AMSOIL synthetic lubricants in place of the manufacturer-branded oil does not void a new vehicle or equipment warranty, and the product is backed by the AMSOIL Limited Warranty. Confirm application match for the specific outboard, sterndrive, or inboard with the AMSOIL Vehicle Lookup Guide, compare four-stroke gasoline engine oil options in the AMSOIL Car Oil Guide, then order AMSOIL 10W-40 100% Synthetic Marine Engine Oil.
I buy this oil for my Yamaha f150. This kit is cheaper than the Yamaha oil change kit with a filter. Also it’s fully synthetic which I prefer.
By Kane from NE Florida on July 15, 2026.
Would recommend this to a friend.
....only the Best....
....Amsoil Marine oil is the only choice for me...10/30 is a perfect choice for my Boat....I've been a 50+ year Customer of Amsoil products....only the Best...thankyou Amsoil....
Is marine engine oil different from automotive motor oil?
Yes. Marine four-stroke oil runs against a different operating profile than automotive oil. The duty cycle includes long idle and trolling stretches, sustained wide-open throttle, water ingress from condensation, and long seasonal storage. The formula uses a heavier dose of rust and corrosion inhibitors than typical automotive oil and is certified to the NMMA FC-W specification, which automotive oils are not required to meet. Use a marine-specific oil in a four-stroke marine engine rather than substituting a car oil.
Can a 4-stroke outboard run on automotive 10W-40?
It is not recommended. Four-stroke outboards and sterndrives that specify a 10W-40 marine oil expect FC-W certification, marine-grade rust and corrosion protection, and additive chemistry tuned for fuel dilution and water contamination. AMSOIL 10W-40 100% Synthetic Marine Engine Oil carries NMMA FC-W (CAT) registration FC-00850T and is built specifically for that service.
What if the engine manual calls for 25W-40 or 10W-30 instead of 10W-40?
Always follow the engine manufacturer specification. The 25W-40 grade is a different product designed for warm-water sterndrive and inboard service that calls for the heavier cold-cranking number. For engines that specify a 10W-30 marine oil, use AMSOIL 10W-30 100% Synthetic Marine Engine Oil (WCT), which carries FC-W (CAT) Registration Number FC-00833T from the same product family.
How often should the oil be changed in a 4-stroke outboard or personal watercraft?
Change the oil at the engine or watercraft manufacturer-recommended interval. Marine four-stroke service is more prone to fuel dilution and water contamination than automotive service, so the manufacturer interval should be treated as a hard limit rather than a starting point for extended drains. For inboards and sterndrives the formula supports up to two times the engine manufacturer-recommended interval or one year, whichever comes first.
Does this oil protect the engine during off-season storage?
Yes. Rust and corrosion protection is one of the formula's primary design targets. Potent inhibitors guard cylinder walls, valve train, and bearings during use and through long periods of inactivity, which matters for boats and personal watercraft that sit through a winter layup or extended dock storage.
Can AMSOIL 10W-40 Synthetic Marine Engine Oil be mixed with other oils?
The formula is compatible with other synthetic and conventional motor oils, but mixing shortens its life expectancy and reduces performance benefits. Extended drain intervals are not supported when oils have been mixed, and aftermarket oil additives are not recommended for use with this oil.
Does using AMSOIL synthetic marine oil void a new outboard or sterndrive warranty?
No. Using AMSOIL synthetic lubricants does not void a new vehicle or equipment manufacturer warranty. AMSOIL 10W-40 100% Synthetic Marine Engine Oil is Warranty Secure and is also backed by the AMSOIL Limited Warranty.
