Short answer
API GL-5 is written for hypoid axle gears under shock loads and high torque, while API GL-4 describes oils for moderate-duty axles and is commonly specified for manual transmissions and transaxles. GL-5 is the only one API lists as in current use, but a higher designation is not automatically a substitute. Follow the designation and viscosity grade in your manual.
Different gears, different jobs
API 1560 describes GL-4 oils for spiral-bevel axles under moderate to severe conditions, hypoid axles under moderate conditions, and selected manual transmissions and transaxles. GL-5 is described for gears, particularly hypoid gears, in axles under high-speed shock load and low-speed high-torque conditions.
The practical split is that GL-5 shows up most often in differentials and axles, while GL-4 shows up most often in synchronized manual gearboxes and some transfer cases and transaxles.
Why GL-5 is not always a drop-in for GL-4
Manual transmissions shift through synchronizers that rely on controlled friction to match gear speeds. Many synchronizer rings have been made from brass or bronze. GL-5 oils use a stronger extreme-pressure additive package for hypoid axles, and some transmission makers specify GL-4 or a dedicated fluid to suit their synchronizer materials and shift feel.
Some gear oils are labeled for both GL-4 and GL-5 service. That dual claim can be appropriate, but it does not replace a transmission maker’s own fluid specification or approval.
Why GL-4 is not a substitute for GL-5
Putting a GL-4 oil in an axle that calls for GL-5 means using an oil described for moderate hypoid service in a unit designed around severe hypoid service. If the manual says GL-5, use GL-5 (and check for any limited-slip requirement).
Status and verification
As of the ninth edition of API 1560 (November 2023), GL-5 is in current use with requirements defined in ASTM D7450, while GL-4 is listed as not in current use because its performance-verification test equipment is no longer available. GL-4 remains common on labels, and API advises end users to ask suppliers for supporting documentation.
Viscosity is separate from both: a 75W-90 oil could be GL-4, GL-5 or both, so match the SAE J306 grade as well as the designation.
Frequently asked questions
Can I mix GL-4 and GL-5 gear oil?
Mixing is not a substitute for using the specified fluid. Drain and refill with the oil your manual lists if you are unsure what is in the unit.
My transmission says GL-4 but the store only has GL-5. What should I do?
Check the transmission maker’s guidance. Some allow a dual-rated GL-4/GL-5 oil; others call for GL-4 or a dedicated manual transmission fluid only.
Which one goes in a rear differential?
Most hypoid rear differentials call for GL-5, often with a limited-slip requirement on vehicles so equipped. Check the manual or the axle tag.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].