About this item

• Product Overview

  Protects Against Wear
  Marine engines operate under high loads and rpm for extended periods. On average, a marine engine powering a boat 30 mph runs at about 5,000 rpm. In contrast, an automotive engine powering a car 60 mph runs at about 2,000 rpm. Higher stress causes traditional automotive oils to break down and lose viscosity. AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is designed with excellent shear stability to deliver a consistent lubricating film. It helps guard engine components from wear for maximum life.

  Excellent Anti-Rust Performance
  Marine engines are prone to rust and corrosion due to constant exposure to humid, wet environments. Rust on engine parts acts like sandpaper, scouring bearings, the cylinder wall, piston rings and other components, while aluminum pistons and other parts can corrode. Rust and corrosion reduce engine power and eventually cause catastrophic failure.

  Good rust protection is not natural to motor oil; it comes only by design. AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is fortified with potent inhibitors that guard against damaging rust and corrosion.

  • Excellent high-stress, high-rpm endurance
  • Increased rust and corrosion protection

  NMMA Certified
  AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is certified by the National Marine Manufacturers Association (NMMA) for use in four-stroke outboards, inboards, stern-drives (inboard/outboards) and PWC. This high-quality oil demonstrates outstanding performance and exceeds the NMMA "FC-W" (Four Cycle Water) specification requirements.

  • FC-W (CAT)^® Registration Number: FC-11825T

  * All trademarked names and images are the property of their respective owners and may be registered marks in some countries. No affiliation or endorsement claim, express or implied, is made by their use. All products advertised here are AMSOIL-engineered for use in the applications shown.

  APPLICATIONS
  Use in gasoline-fueled four-stroke inboards, outboards, inboard/outboards and personal watercraft that require any of the specifications listed below, including those made by Mercury* and Mercruiser*.

  • API SM
  • NMMA FC-W Catalyst Compatible

  SERVICE LIFE
  Four-stroke outboard motors and personal watercraft: Due to potential fuel dilution or water contamination, change oil at the engine or watercraft manufacturer-recommended interval. Inboard and I/O applications: Change oil at two times the engine manufacturer-recommended interval, or once a year, whichever comes first.

  WARRANTY
  AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is Warranty Secure^®, keeping your factory warranty intact. It is a high-performance replacement for vehicle manufacturer-branded products and is also backed by the AMSOIL Limited Warranty (G1363). For details, visit www.amsoil.com/warranty/.

  HEALTH & SAFETY
  This product is not expected to cause health concerns when used for the intended application and according to the recommendations in the Safety Data Sheet (SDS). And SDS is available via the Internet at www.amsoil.com or upon request at (715) 392-7101. Keep Out of Reach of Children. Recycle used oil and bottle.

• Technical Specifications
  Data Bulletin

  Data Bulletins

  Warranty

  WARRANTY

  SDS

  SDS

  
  
  
• Product Reviews
  4.9 star rating

  Based on 36 reviews

  Based on 36 reviews

  See all customer reviews

  Sort by

  Most Recent Most Helpful Highest Rating Lowest Rating Oldest

  • 5 star rating
    Outstanding product
    By  Chuck,  a  Verified Buyer  from  Pa  on  April 29, 2026

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    This oil immediately cured the low oil pressure problem in my outboard. Motor sounds great, runs great. The lower end also performs well with the 75-90 Marine Amsoil gear lube. I run Amsoil in every motor I own including those in my cars from the forties and fifties and I have for decades. Great stuff.
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  • 5 star rating
    Great Product at a Great Price!
    By  JARED,  a  Verified Buyer  on  March 15, 2026

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    Been using Amsoil in my truck and my boat and love the product. 2021 Chevy Trail Boss, 1966 Bertram 25" twin 3.7L Mercruisers. Going to start using the marine gear oil next.
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Quality product
    By  ROBERT,  a  Verified Buyer  from  Windsor  on  October 21, 2025

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    Dhipping was fast, price was fantastic, used this their products in my boat and motorcycle. No isdues.
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Mercury Marine 250 horse IO 2020 model
    By  DAVID,  a  Verified Buyer  from  Hudson Wisconsin  on  October 19, 2025

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    Switched from Mercury Marine 25/40 synthetic blend to amsoil Marine 25/40 blend over the last several years I've been switching everything over to Amsoil a little bit more money given the price of automobile engines truck engines and Marine engines it's a small price to pay for top quality product they always have fast shipping very happy and you know where the product comes from Superior Wisconsin yay USA made
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Out with the original in with Amsoil
    By  RANDY,  a  Verified Buyer  from  Hartland Mich  on  July 30, 2025

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    Break in complete on new 2025 Mercury 90 outboard Out with the Mercury oil, in with Amsoil. c”,)
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Fast shipping
    By  MATTHEW,  a  Verified Buyer  on  April 06, 2025

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    Shipped immediately was extremely happy
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    25W-40 Marine oil
    By  KIERAN,  a  Verified Buyer  from  Long Island NY  on  November 23, 2024

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    Used this oil in my Mercruiser 350's for 1 season So far so good . Better value than mercruiser's oil and easy to locate and buy. Definitely using from now on.
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Excellent and good price
    By  STEVEN,  a  Verified Buyer  from  Quebec  on  November 01, 2024

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    Very good Martine engine oil for mercruiser engines, only upgrade that could be made is having 1 liter bottles instead of 1quart
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Makes the Merc purr
    By  BRIAN,  a  Verified Buyer  on  October 29, 2024

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    Good quality oil
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Equal or superior
    By  TIM,  a  Verified Buyer  on  May 10, 2024

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    Seems on par or superior to mercury oil. Will see next year when I change it but maintaining warranty and half the price of the alternative and a brand I trust is with the try.
    Bottom Line

    Yes, I would recommend this to a friend.
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• Highlights

  • Excellent high-stress, high-rpm endurance
  • Increased rust and corrosion protection
• Returns

  We offer easy returns for customers. If your product is unused and unopened, you may return it within 30 days for a full refund of the purchase price. View full AMSOIL Return Policy.

• Payments

  We accept Visa, Mastercard, American Express, Discover and PayPal, and provide secure checkout and encrypted payment processing.

• Product Overview

  Protects Against Wear
  Marine engines operate under high loads and rpm for extended periods. On average, a marine engine powering a boat 30 mph runs at about 5,000 rpm. In contrast, an automotive engine powering a car 60 mph runs at about 2,000 rpm. Higher stress causes traditional automotive oils to break down and lose viscosity. AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is designed with excellent shear stability to deliver a consistent lubricating film. It helps guard engine components from wear for maximum life.

  Excellent Anti-Rust Performance
  Marine engines are prone to rust and corrosion due to constant exposure to humid, wet environments. Rust on engine parts acts like sandpaper, scouring bearings, the cylinder wall, piston rings and other components, while aluminum pistons and other parts can corrode. Rust and corrosion reduce engine power and eventually cause catastrophic failure.

  Good rust protection is not natural to motor oil; it comes only by design. AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is fortified with potent inhibitors that guard against damaging rust and corrosion.

  • Excellent high-stress, high-rpm endurance
  • Increased rust and corrosion protection

  NMMA Certified
  AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is certified by the National Marine Manufacturers Association (NMMA) for use in four-stroke outboards, inboards, stern-drives (inboard/outboards) and PWC. This high-quality oil demonstrates outstanding performance and exceeds the NMMA "FC-W" (Four Cycle Water) specification requirements.

  • FC-W (CAT)^® Registration Number: FC-11825T

  * All trademarked names and images are the property of their respective owners and may be registered marks in some countries. No affiliation or endorsement claim, express or implied, is made by their use. All products advertised here are AMSOIL-engineered for use in the applications shown.

  APPLICATIONS
  Use in gasoline-fueled four-stroke inboards, outboards, inboard/outboards and personal watercraft that require any of the specifications listed below, including those made by Mercury* and Mercruiser*.

  • API SM
  • NMMA FC-W Catalyst Compatible

  SERVICE LIFE
  Four-stroke outboard motors and personal watercraft: Due to potential fuel dilution or water contamination, change oil at the engine or watercraft manufacturer-recommended interval. Inboard and I/O applications: Change oil at two times the engine manufacturer-recommended interval, or once a year, whichever comes first.

  WARRANTY
  AMSOIL 25W-40 Synthetic Blend Marine Engine Oil is Warranty Secure^®, keeping your factory warranty intact. It is a high-performance replacement for vehicle manufacturer-branded products and is also backed by the AMSOIL Limited Warranty (G1363). For details, visit www.amsoil.com/warranty/.

  HEALTH & SAFETY
  This product is not expected to cause health concerns when used for the intended application and according to the recommendations in the Safety Data Sheet (SDS). And SDS is available via the Internet at www.amsoil.com or upon request at (715) 392-7101. Keep Out of Reach of Children. Recycle used oil and bottle.

• Technical Specifications
  Data Bulletin

  Data Bulletins

  Warranty

  WARRANTY

  SDS

  SDS

  
  
  
• Product Reviews
  4.9 star rating

  Based on 36 reviews

  Based on 36 reviews

  See all customer reviews

  Sort by

  Most Recent Most Helpful Highest Rating Lowest Rating Oldest

  • 5 star rating
    Outstanding product
    By  Chuck,  a  Verified Buyer  from  Pa  on  April 29, 2026

    Open report issue form

    Describe the issue

    Failed to verify captcha

    Cancel Submit
    This oil immediately cured the low oil pressure problem in my outboard. Motor sounds great, runs great. The lower end also performs well with the 75-90 Marine Amsoil gear lube. I run Amsoil in every motor I own including those in my cars from the forties and fifties and I have for decades. Great stuff.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Great Product at a Great Price!
    By  JARED,  a  Verified Buyer  on  March 15, 2026

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    Describe the issue

    Failed to verify captcha

    Cancel Submit
    Been using Amsoil in my truck and my boat and love the product. 2021 Chevy Trail Boss, 1966 Bertram 25" twin 3.7L Mercruisers. Going to start using the marine gear oil next.
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Quality product
    By  ROBERT,  a  Verified Buyer  from  Windsor  on  October 21, 2025

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    Dhipping was fast, price was fantastic, used this their products in my boat and motorcycle. No isdues.
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Mercury Marine 250 horse IO 2020 model
    By  DAVID,  a  Verified Buyer  from  Hudson Wisconsin  on  October 19, 2025

    Open report issue form

    Describe the issue

    Failed to verify captcha

    Cancel Submit
    Switched from Mercury Marine 25/40 synthetic blend to amsoil Marine 25/40 blend over the last several years I've been switching everything over to Amsoil a little bit more money given the price of automobile engines truck engines and Marine engines it's a small price to pay for top quality product they always have fast shipping very happy and you know where the product comes from Superior Wisconsin yay USA made
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Out with the original in with Amsoil
    By  RANDY,  a  Verified Buyer  from  Hartland Mich  on  July 30, 2025

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    Describe the issue

    Failed to verify captcha

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    Break in complete on new 2025 Mercury 90 outboard Out with the Mercury oil, in with Amsoil. c”,)
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Fast shipping
    By  MATTHEW,  a  Verified Buyer  on  April 06, 2025

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    Shipped immediately was extremely happy
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    25W-40 Marine oil
    By  KIERAN,  a  Verified Buyer  from  Long Island NY  on  November 23, 2024

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    Used this oil in my Mercruiser 350's for 1 season So far so good . Better value than mercruiser's oil and easy to locate and buy. Definitely using from now on.
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Excellent and good price
    By  STEVEN,  a  Verified Buyer  from  Quebec  on  November 01, 2024

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    Very good Martine engine oil for mercruiser engines, only upgrade that could be made is having 1 liter bottles instead of 1quart
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)
  • 5 star rating
    Makes the Merc purr
    By  BRIAN,  a  Verified Buyer  on  October 29, 2024

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    Good quality oil
    Bottom Line

    Yes, I would recommend this to a friend.
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  • 5 star rating
    Equal or superior
    By  TIM,  a  Verified Buyer  on  May 10, 2024

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    Seems on par or superior to mercury oil. Will see next year when I change it but maintaining warranty and half the price of the alternative and a brand I trust is with the try.
    Bottom Line

    Yes, I would recommend this to a friend.
    Was this review helpful?

    YES (0) NO (0)