Axles, manual transmissions and transfer cases need lubricants that stand up to sliding contact between gear teeth. The bottle may show a grade such as 75W-90 or 80W-90, plus a service designation such as API GL-5. Those labels are not the same as motor oil grades, even when the numbers look similar.
SAE gear oil grades (J306)
Automotive gear oil grades follow the SAE J306 viscosity classification. Like motor oil, a multi-grade such as 75W-90 has a cold grade (75W) and a hot grade (90). The test temperatures and limits differ from the motor oil system (SAE J300), so a “90” gear oil is not the same thickness as an SAE 90 motor oil would be, and you should not treat the numbers as interchangeable.
- Lower W numbers are graded for better flow when cold, which can help with cold shifting and quicker lubrication at start-up.
- Higher hot numbers (90, 140) are thicker grades at operating temperature under the SAE gear oil tests.
GL-4 and GL-5, in general terms
API gear oil service designations describe the kind of gear duty the lubricant is formulated for. At a high level:
- GL-4 lubricants are commonly associated with certain manual transmissions and spiral-bevel gears under moderate to severe conditions, as defined by the API category.
- GL-5 lubricants are commonly associated with hypoid gears in axles under high-speed or high-load conditions, as defined by the API category.
Some gearboxes are sensitive to the additive chemistry in a GL-5 fluid, and some axles require GL-5. That is why the owner’s manual and the fill-plug label matter more than a general rule of thumb. Limited-slip differentials may also require a fluid with friction modifiers, or an additive, if the manual says so.
How to choose
- Read the manual for viscosity, API service designation and any limited-slip or manufacturer specs.
- Check the fill plug or differential tag if the vehicle has one.
- Look the vehicle up in AMSOIL’s official lookup guide or the vehicle lookup to see which AMSOIL gear lubricants are listed.
Boat lower units often use marine gear lube with different requirements; see the outboard winterizing checklist and AMSOIL’s outboard lookup.
Frequently asked questions
Is 75W-90 gear oil the same thickness as 5W-90 motor oil?
No. Gear oils and motor oils use different SAE viscosity classification systems (J306 vs J300). Do not swap them based on similar-looking numbers.
Can I put GL-5 in a transmission that calls for GL-4?
Follow the owner’s manual. Some transmissions specify GL-4 only. Using a different service designation than the manufacturer lists can cause problems.
What does limited-slip mean for gear oil?
Some limited-slip differentials need a fluid with friction modifiers, or a separate additive, so the clutches work quietly. Your manual says whether that applies.
AMSOIL gear lubricants
- AMSOIL SEVERE GEAR 80W-90 100% Synthetic Gear Lube. AMSOIL says it is “engineered with high film strength for high-load demands,” “reduces friction and provides the ultimate protection against wear,” and is “especially well-suited for towing, hauling, racing, commercial use or other severe duty.” AMSOIL also lists “contains friction modifiers for limited-slip applications” among its highlights. Confirm the viscosity and specs your axle or transmission requires.
- AMSOIL 75W-90 100% Synthetic Marine Gear Lube is listed for marine lower units that require 75W-90 or 80W-90 gear lube. See the outboard page for storage-related notes.
Product wording above is quoted or summarized from amsoil.com. Read each product’s label and data sheet for directions.
Always match the manual, then confirm with AMSOIL’s official lookup guide.