API GL-5 and SAE viscosity describe two different properties of automotive gear oil. API GL-5 identifies a performance category intended for demanding gear applications, including many hypoid axles; the SAE grade describes the lubricant’s viscosity behavior at specified temperatures. Neither specification replaces the other, and the correct product must meet both the vehicle or equipment maker’s performance requirement and its specified viscosity grade. At Wanxiang, we recommend checking the owner’s manual or component documentation before selecting or sourcing gear oil.
API GL-5 is a service designation used for gear lubricants intended for applications with substantial contact pressure and sliding, such as many hypoid gears in vehicle axles. These operating conditions can place demanding loads on gear teeth, so GL-5 formulations typically use extreme-pressure (EP) chemistry to help protect surfaces. The designation indicates a performance category, not a viscosity grade or a guarantee that the oil suits every transmission or axle.
For buyers, the practical question is whether the equipment manufacturer calls for API GL-5 or another specific performance level. Do not assume that a product with a higher-sounding or different API category is automatically a suitable replacement. Synchronizer materials and design can affect compatibility, so a GL-5 oil should not be substituted for a GL-4 product unless the equipment maker permits it.
SAE gear-oil viscosity grades are defined by SAE J306. A multigrade such as 75W-90 combines a winter grade, marked by “W,” with a higher-temperature grade. The winter designation relates to low-temperature viscosity requirements; the second number identifies a viscosity range measured at higher temperature.
Gear-oil grades under SAE J306 are not directly interchangeable with engine-oil grades under SAE J300. For example, 75W-90 gear oil does not have the same viscosity meaning as an engine oil carrying a similar-looking SAE number. Always confirm that a specification refers to gear oil and the correct SAE standard.
In 75W-90, “75W” indicates the oil’s qualification for a defined low-temperature viscosity class, while “90” indicates its kinematic viscosity class at high temperature. The two numbers are not a simple measure of viscosity increase, and they do not indicate that the oil changes from one fixed thickness to another. They provide standardized information that helps users compare products for an application.
As examples of SAE J306 grade limits, an SAE 75W oil has a maximum low-temperature viscosity of 150,000 mPa·s at -40°C. An SAE 90 grade has a kinematic viscosity of 13.5 to less than 18.5 mm²/s at 100°C, while SAE 140 has a range of 24.0 to less than 32.5 mm²/s at 100°C. These figures illustrate why the second number matters, but they do not establish which grade is right for a specific vehicle.
| Specification | What it describes | What buyers should confirm |
|---|---|---|
| API GL-5 | Gear-lubricant service performance category | That the component maker calls for GL-5 and permits the formulation |
| SAE 75W-90 | Gear-oil viscosity grade across defined temperature conditions | That the specified viscosity grade matches the application |
| SAE 75W-140 | A different multigrade viscosity classification | That the equipment maker approves it rather than assuming thicker is better |
API GL-5 and SAE viscosity answer separate questions. GL-5 helps describe the performance level required for a gear system, while SAE J306 helps describe viscosity under specified conditions. A product may carry both designations, such as API GL-5 SAE 75W-90, but each designation must independently match the equipment requirement.
Wanxiang Product Page
Application conditions matter, but they do not override the manufacturer’s instructions. A colder operating environment may make the low-temperature behavior of the approved grade especially relevant, while load and operating design influence the required performance category. Avoid selecting a lubricant solely from a vehicle’s age, climate, or a general rule of thumb.
Start with the owner’s manual, service documentation, axle identification, or equipment manufacturer’s specification. Record the required API category, SAE J306 grade, and any additional product or material compatibility requirements. If the documentation lists multiple acceptable grades, follow its stated temperature or operating conditions when choosing among them.
Confirm that the product label or technical data sheet identifies the required gear-oil performance category and viscosity grade. If a specification is unclear, ask the supplier for current technical documentation and clarify whether stated performance is formally approved, claimed, or simply suitable for consideration. Those terms can have different meanings, so buyers should not treat them as interchangeable.
For transmissions with synchronizers, limited-slip differentials, or specialized materials, verify any additional compatibility requirement with the equipment maker. Buyers sourcing for fleets or distribution should also define the required package sizes, destination market, labeling language, and expected order volume. These details help a supplier assess whether the requested product and supply arrangement can be supported.
For B2B sourcing, provide the intended application, requested API category, SAE J306 grade, target market, packaging preference, and any labeling or documentation needs. At Wanxiang, we can use those details to discuss product selection and clarify the information available for a requested gear-oil specification. Product availability, customization, and commercial terms should be confirmed for each inquiry rather than assumed.
To evaluate a supplier, ask for a current technical data sheet and check that the product description aligns with your equipment requirement. Clarify packaging options, minimum order quantity, lead time, and export documentation during quotation, as these depend on the specific product and order. This process helps procurement teams compare offers on specification and supply fit, not price alone.
API GL-5 and SAE viscosity are complementary specifications, not alternatives: GL-5 addresses gear-service performance, while the SAE J306 grade indicates viscosity behavior. To choose automotive gear oil, first confirm the component maker’s required API category and viscosity grade, then verify compatibility and supplier documentation. If you are sourcing for a fleet, distributor, or OEM program, send Wanxiang your application and target specifications to begin a product and supply discussion.
If you want to learn more, please visit our website API GL-5 and SAE Viscosity: Understanding Automotive Gear Oil Specifications.