Short answer
API GL-5 is the American Petroleum Institute designation for gear lubricants intended for gears, particularly hypoid gears, in axles under high-speed shock loads and low-speed, high-torque conditions. It is the only API gear oil designation currently in use, and its performance requirements are defined in ASTM D7450. Limited-slip friction requirements are left to the axle manufacturer.
What hypoid gears demand
In a hypoid axle, the pinion gear sits below the centerline of the ring gear. That offset lets the driveshaft sit lower, but it also makes the gear teeth slide across each other as well as roll, which raises contact pressure and heat. Under shock loads, such as towing launches or wheel hop, or under low-speed high torque, the metal surfaces can scuff or score without an extreme-pressure additive film.
GL-5 oils are formulated for those conditions. That is why rear axles and many front differentials on trucks, SUVs and cars list GL-5 in the owner’s manual.
How GL-5 is defined
The ninth edition of API 1560, published in November 2023, lists GL-5 as the only designation in current use and points to the most recent version of ASTM D7450 for its performance specifications. GL-1, GL-2, GL-3, GL-4, GL-6 and MT-1 are listed as not in current use.
API 1560 also states that viscosity grade is separate from performance level: gear oils are identified by an SAE J306 viscosity grade, such as 75W-90 or 80W-140, in addition to GL-5.
GL-5 versus SAE J2360
SAE J2360 is a gear lubricant standard for commercial and military use that includes the GL-5 test requirements as its core and adds more. API notes that J2360 includes requirements for elastomer (seal) compatibility and gear cleanliness after oxidation that GL-5 does not. Oils qualified to J2360 are reviewed and listed on a Qualified Products List administered by the Lubricant Review Institute.
Heavy-duty axle makers and fleets often specify J2360 or their own approval rather than GL-5 alone. If your manual names J2360 or an axle-maker specification, look for that in the product data.
What GL-5 does not cover
GL-5 does not define limited-slip friction performance; API leaves that to the axle manufacturer, which is why some limited-slip axles specify an oil with friction modifiers or an added friction modifier. SAE J2360 notes that these lubricants are not appropriate for worm gears. Many synchronized manual transmissions call for GL-4 or a dedicated manual transmission fluid instead, so GL-5 is not a universal gear oil.
Frequently asked questions
Is GL-5 better than GL-4?
GL-5 is designed for more severe hypoid axle service. It is not automatically the right choice for a manual transmission that calls for GL-4. Use the designation in the manual.
Does GL-5 mean an oil is safe for limited-slip differentials?
No. GL-5 leaves limited-slip friction requirements to the axle maker. Check whether your axle needs a limited-slip rated oil or added friction modifier.
What does 75W-90 GL-5 mean?
75W-90 is the SAE J306 viscosity grade and GL-5 is the API performance designation. Both need to match the manual.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
