AMSOIL 75W-90 Synthetic Marine Gear Lube protects lower units, sterndrives, and marine transmissions with extreme-pressure protection that holds up even with water contamination. It guards against rust and corrosion in both freshwater and saltwater service.
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Product data sheets
View, download, or save a copy of the Product Data Sheet or Safety Data Sheet for AMSOIL 75W-90 100% Synthetic Marine Gear Lube.
Service Marine Lower Units to Prevent Water Damage
Available sizes
Product code
Package size
Unit of measure
AGMTB-EA
10 oz. Tube
10 oz. Tube
AGMPK-EA
Quart Easy-Pack
Each
AGMQT-EA
Quart
Quart
AGMPK-CA
Case of 12 Quart Easy-Packs
Case of 12
AGM05-EA
5 Gallon Pail
5 Gallon Pail
AGM16-EA
16 Gallon Keg
16 Gallon Keg
AGM55-EA
55 Gallon Drum
55 Gallon Drum
AMSOIL 75W-90 100% Synthetic Marine Gear Lube
AMSOIL 75W-90 100% Synthetic Marine Gear Lube (AGM) is a marine-specific lower-unit gear oil for outboards, sterndrives, I/O drives, V-drives, bow thrusters, and marine transmissions. The formula combines synthetic base oils with extreme-pressure additives to handle shock loading from propeller cavitation and keeps protecting gears and bearings when the gear case ingests water through a damaged seal. It meets API GL-4 and GL-5 and works in both fresh- and saltwater service.
Extreme-pressure protection for shock loading
Heavily loaded outboards and high-horsepower sterndrives put repeated shock loads through the lower unit when the propeller cavitates and reloads. Synthetic Marine Gear Lube is fortified with extreme-pressure (EP) additives so the gear oil holds a strong protective film at the gear-tooth contact point during those load spikes. In Falex Procedure B testing (ASTM D3233), the formula records a 2000 LbF failure load, and the same 2000 LbF result is reported with 15 percent water contamination, showing the EP additive system stays effective when water is present in the gear case.
Water-emulsion resistance and gear wear protection
A lower-unit gear case can pick up water through a worn prop-shaft seal, a damaged shift-shaft seal, or a cracked housing. Conventional gear oils tend to lose film strength when contaminated, letting metal-on-metal wear take over. Synthetic Marine Gear Lube is engineered to deliver advanced outboard protection against power loss and gear wear with up to 15 percent water contamination, based on AMSOIL testing in ASTM D3233 and ASTM D892. The result is steady protection through the season, even when a seal starts to leak.
Foam control and oil-film stability
Foam in a gear oil disrupts the lubricating film and accelerates wear, and water contamination raises the risk of foaming. In ASTM D892 foam-stability testing using the 20/50/20 maximum criterion, Synthetic Marine Gear Lube records 0/0/0 across Sequences I, II, and III, both new and after water exposure. Zero measured foam under industry-standard testing keeps the oil film intact at the gear and bearing surfaces during high-rpm operation.
Rust, corrosion, and seal protection
Marine gear cases live in a humid, often corrosive environment. The formula is designed to prevent rust on internal steel surfaces and is compatible with the aluminum, copper, and brass alloys used in modern lower units, so it can be run in mixed-metal gear cases without fear of corroding bearing cages, shift forks, or propeller-shaft hardware. The additive package also promotes long seal life, which helps prevent water intrusion at the prop-shaft and shift-shaft seals over a longer service window.
Fresh- and saltwater outboard, sterndrive, and I/O coverage
Synthetic Marine Gear Lube is approved for fresh- and saltwater applications that require a 75W-90 or 80W-90 gear lube. It serves as a synthetic replacement for OEM-branded marine gear oils on lower units made by Mercury/Mariner, Johnson/Evinrude, Bombardier, Honda, Yamaha, Suzuki, Force/US Marine, Nissan, Tohatsu, and Harbormaster, sterndrives made by Mercruiser-Bravo, OMC/Bombardier, Volvo-Penta, Yanmar, Konrad, and Marine Drive Systems, V-drives made by Casale and Menkins, bow thrusters made by Harbormaster Marine, and transmissions made by ZF Marine Transmission. All trademarked names are the property of their respective owners, and no affiliation or endorsement claim is made by their use.
Recommended applications and service intervals
AMSOIL 75W-90 Synthetic Marine Gear Lube is engineered for marine lower-unit gear cases and other marine drive components that call for a 75W-90 or 80W-90 gear oil meeting API GL-4 or GL-5. Service interval guidance follows the boat builder and engine manufacturer schedule:
Outboard lower units: follow the engine manufacturer recommended gear-oil service interval and inspection schedule.
Sterndrives, I/O, V-drives, bow thrusters, and marine transmissions: follow the OEM service interval and check for milky or contaminated oil at the recommended inspection points.
End-of-season service: changing the gear lube at the end of the boating season, prior to winter storage, is generally a good practice for marine applications.
Outside US and Canada: follow the engine manufacturer specification for gear-oil viscosity and API service classification.
The formula is compatible with other commercially available conventional and synthetic marine gear lubes, so it can be used to top off an existing fill in the field without flushing. Aftermarket gear-oil additives are not required and should be avoided unless specifically called for by the engine manufacturer. For a viscosity and OEM cross-reference for a specific outboard, sterndrive, or marine transmission, use the AMSOIL Vehicle Lookup Guide.
Typical technical properties
Property
Value
Kinematic Viscosity at 100 C, cSt (ASTM D445)
15.9
Kinematic Viscosity at 40 C, cSt (ASTM D445)
119.4
Viscosity Index
142
Flash Point, C (F) (ASTM D92), 150 C minimum
204 (399)
Pour Point, C (F) (ASTM D97)
-39 (-38)
Falex Procedure B (ASTM D3233), failure load, LbF
2000
Foam Stability (ASTM D892), 20/50/20 maximum
0/0/0
Order AMSOIL 75W-90 Synthetic Marine Gear Lube
AMSOIL 75W-90 100% Synthetic Marine Gear Lube is available in 10 oz. tube (AGMTB-EA), Quart Easy-Pack (AGMPK-EA), Quart bottle (AGMQT-EA), case of 12 Quart Easy-Packs (AGMPK-CA), 5 gallon pail (AGM05-EA), 16 gallon keg (AGM16-EA), and 55 gallon drum (AGM55-EA). Synthetic Marine Gear Lube is Warranty Secure and is a high-performance replacement for manufacturer-branded marine gear oils, and the product is also covered by the AMSOIL Limited Warranty. Confirm application match for a specific outboard, sterndrive, or marine transmission with the AMSOIL Vehicle Lookup Guide, pair the lower-unit service with two-stroke or four-stroke marine engine oil from the Marine 2-Stroke Oil Guide, and order AMSOIL 75W-90 100% Synthetic Marine Gear Lube.
If you want to keep your lower unit intact you have to either run a quality oil or change it very frequently. This gear oil gives me confidence in knowing I will be protected until my next oil change vs changing lower unit oil 4x per year with the cheap stuff.
By Sam from Ohio on January 18, 2026.
Would recommend this to a friend.
Superior lubrication and water intrusion protection.
Don’t go boating without it.
By K Lamar from Chattanooga on November 25, 2025.
Would recommend this to a friend.
Outboard
I have a 2019 Suzuki 90hp and a 1994 Mercury 115hp and use Amsoil gear lube in each one. Thanks
Can AMSOIL 75W-90 Marine Gear Lube replace Mercury High Performance Gear Lube?
Yes. The product is a synthetic replacement for OEM-branded marine gear oils on lower units made by Mercury/Mariner, including applications that call for a 75W-90 or 80W-90 marine gear lube meeting API GL-4 or GL-5. The same approval set covers Mercruiser-Bravo sterndrives. Confirm the exact viscosity and specification listed in the engine owner manual before service.
What does API GL-4 and GL-5 mean for a marine gear lube?
GL-4 and GL-5 are API gear-oil service categories that describe extreme-pressure performance for hypoid and spiral-bevel gearing under shock and high-load conditions. A lower-unit gear case relies on those EP properties to handle propeller torque spikes. Synthetic Marine Gear Lube meets both GL-4 and GL-5, covering the OEM call-outs across outboard, sterndrive, and marine transmission applications, including Yamaha, Honda, Suzuki, Tohatsu, Johnson/Evinrude, and Force/US Marine outboards.
Is it safe to use in saltwater service?
Yes. The product is designed for both fresh- and saltwater applications. The additive package promotes rust protection on internal steel surfaces, is compatible with aluminum, copper, and brass alloys in mixed-metal gear cases, and supports long seal life to help limit water intrusion at the prop-shaft and shift-shaft seals.
How does it perform when water gets into the lower unit?
Falex Procedure B testing (ASTM D3233) reports a 2000 LbF failure load both with no water and with 15 percent water contamination, and ASTM D892 foam-stability testing returns 0/0/0 against the 20/50/20 maximum, including under water exposure. The formula is engineered to keep delivering EP and anti-wear protection with up to 15 percent water contamination, based on AMSOIL testing of 75W-90 Synthetic Marine Gear Lube in ASTM D3233 and ASTM D892. A seal that is leaking water still needs to be repaired, but the gear oil keeps protecting the gears in the meantime.
How often should marine gear lube be changed?
Follow the engine manufacturer recommended service interval for marine gear oil and inspect at the schedule shown in the owner manual. As a general practice, changing gear lube at the end of the boating season, prior to winter storage, is recommended. Inspect the drained oil for a milky appearance, which can indicate water intrusion that requires seal service.
Can Synthetic Marine Gear Lube be mixed with conventional or other synthetic marine gear oils?
The formula is compatible with other commercially available conventional and synthetic marine gear lubes, so it can be added to an existing fill without flushing. For best results, complete the next scheduled gear-oil change with a full Synthetic Marine Gear Lube fill so the additive system is not diluted.
Does running this gear lube void a new outboard or sterndrive warranty?
No. Using AMSOIL synthetic lubricants does not void a new boat or engine manufacturer warranty, and the product is Warranty Secure and covered by the AMSOIL Limited Warranty. Use the viscosity grade and API service category listed in the engine owner manual.
