The right synthetic gear oil matches the viscosity your owner’s manual calls for (like 75W-90 or 75W-140) and the API rating (GL-5 for most differentials and axles, GL-4 for many manual transmissions). AMSOIL makes three main gear oil families: SEVERE GEAR for towing and hard use, Long Life for everyday driving and extended service, and Manual Transmission & Transaxle Gear Lube for GL-4 gearboxes.
Before you buy: the front axle, rear axle, transfer case and manual transmission on the same vehicle can each take a different gear oil. Look up your exact vehicle to see the right AMSOIL gear oil and capacity for each one.
GL-5 vs. GL-4: Why It Matters
Differentials and axles use hypoid gears that slide under heavy load, so they need the extra extreme-pressure additives in a GL-5 oil. Many manual transmissions and front-wheel-drive transaxles have brass or bronze synchronizers designed around GL-4. The wrong oil can mean worn differential gears or notchy, hard shifts. Some manual transmissions call for ATF or a dedicated manual-transmission fluid instead of gear oil.
AMSOIL SEVERE GEAR Synthetic Gear Lube (Towing, Hauling, Off-Road)
SEVERE GEAR is AMSOIL’s toughest gear oil, made for trucks that tow, haul, plow, race or go off-roading. Its additives form an iron-sulfide barrier on gear surfaces to fight wear, pitting and scoring, and it resists the heat spikes that break down gear oil under heavy load. It contains friction modifiers for limited-slip differentials.
Specifications: API GL-5, MT-1, MIL-PRF-2105E, Dana SHAES 234, Mack GO-J.
- 75W-90: replaces 75W-90 and 80W-90. Shop 75W-90
- 75W-110: a heavier option for certain 75W-90 and 80W-90 axles that work hard. Shop 75W-110
- 75W-140: replaces 75W-140, 80W-140 and 85W-140. Shop 75W-140
AMSOIL Long Life Synthetic Gear Lube (Cars, Trucks and Commercial Use)
Long Life is built for extended service in cars, trucks and commercial equipment. Its synthetic base oils resist breakdown, flow well in the cold and stay stable in summer heat, so it works year-round in front and rear differentials. It also helps extend seal life.
- 75W-90: the most common differential grade. Shop 75W-90
- 80W-140: for applications calling for a 140-weight gear oil. Shop 80W-140
AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90 (GL-4)
The gear oil for manual transmissions and transaxles that call for GL-4, protecting the brass and bronze synchronizers for smooth shifts. Use it where the manual specifies 75W-85, 75W-90 or 80W-90 and API GL-4, ZF TE-ML 02B, 16A, 17A or 19A, Chrysler MS-9070, MAN 341-Z2 or MB 235.5.
Quick Pick Guide
| Your situation | Best fit |
|---|---|
| Towing, hauling, lifted truck, off-road, plowing | SEVERE GEAR |
| Daily driver, normal use | Long Life |
| Manual transmission or transaxle calling for GL-4 | Manual Transmission & Transaxle Gear Lube 75W-90 |
| Limited-slip differential | SEVERE GEAR |
Shop AMSOIL Gear Oil
Gear Oil FAQ
Neither by default. Use the grade your owner’s manual lists for towing. For axles that work hard, AMSOIL offers SEVERE GEAR 75W-110 as a heavier option for certain 75W-90 and 80W-90 applications.
Only if the transmission maker allows it. Many manual transmissions are built for GL-4 because of their brass or bronze synchronizers. For those, use AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90.
Yes. SEVERE GEAR contains friction modifiers and works in most limited-slip differentials. A few applications call for adding AMSOIL Slip Lock.
They are compatible, but for full performance, drain the old oil completely before refilling with AMSOIL.
About Art Bourget
Art Bourget is an Authorized AMSOIL Dealer and the person behind Full Synthetic Oil. Based in Central Florida, he helps owners and commercial accounts choose lubricants for how their vehicles actually work, and arranges used-oil analysis through Oil Analyzers, Inc.
Specifications are from AMSOIL product data (SEVERE GEAR bulletin, Long Life product page), checked October 2026. Always confirm the correct grade and API rating for your vehicle with your owner’s manual or AMSOIL’s vehicle lookup. Full Synthetic Oil is an Authorized AMSOIL Dealer.
