GL-4 vs GL-5 Gear Oil: What's the Difference and When to Use Each
Short answer: GL-5 gear oil carries a higher level of extreme-pressure (EP) anti-wear additive than GL-4, roughly double, so it protects high-load, high-speed hypoid gears like rear-axle differentials. GL-4 uses a lower EP additive level suited to manual transmissions and transaxles, where the higher-additive GL-5 chemistry can, in some designs, be aggressive toward brass and bronze (yellow-metal) synchronizers. Always follow the OEM specification for your component.
What GL-4 and GL-5 actually mean
GL-4 and GL-5 are API gear-oil service categories that define a level of performance and EP protection; they are not viscosity grades. A gear oil can be SAE 75W-90, 80W-90, or 85W-140 and be rated GL-4, GL-5, or both. The GL rating tells you the load-carrying capability; the SAE grade tells you the viscosity.
The core difference: EP additive level
- GL-5 uses a higher concentration of sulfur-phosphorus EP additives for the high contact pressures of hypoid gearsets.
- GL-4 uses about half that EP additive level, which is ample for manual gearboxes and spiral-bevel gears.
Where each is used
- GL-5: hypoid differentials, rear axles, heavy-duty drive axles, and high-load or high-shock applications.
- GL-4: manual transmissions and transaxles that specify GL-4, especially those with yellow-metal synchronizers.
The yellow-metal compatibility issue
The active sulfur-phosphorus chemistry that makes GL-5 so protective can, under heat, be corrosive to brass and bronze synchronizers in some manual transmissions. That is why many manual-transmission makers specify GL-4 while differentials specify GL-5. Some modern GL-5 oils are engineered to be yellow-metal safe or are dual-rated GL-4/GL-5, but never assume; match the OEM requirement.
Is GL-5 backward compatible with GL-4?
GL-5 generally meets or exceeds GL-4 load-carrying performance, but it is not always a safe drop-in substitute in a GL-4 application because of the yellow-metal concern. If the manual specifies GL-4, use a GL-4 (or a verified GL-4/GL-5 dual-rated) oil.
How to choose
- Find the OEM-specified GL rating in the owner's manual or service spec.
- Match the SAE viscosity grade (for example, 75W-90).
- For manual transmissions with yellow-metal parts, confirm GL-4 or a yellow-metal-safe GL-5.
- For differentials and axles, use GL-5.
Ultra1Plus gear oils
- Gear oil: Shop gear oils
Need help matching a spec? Check the product page or contact our team. Bulk drums and totes available.
Frequently asked questions
What is the difference between GL-4 and GL-5 gear oil?
GL-5 has roughly double the extreme-pressure (EP) additive of GL-4, protecting high-load hypoid gears like differentials. GL-4 has a lower EP level suited to manual transmissions, where GL-5's chemistry can harm yellow-metal synchronizers. Match the OEM spec.
Can I use GL-5 instead of GL-4?
Not always. GL-5 meets or exceeds GL-4 load-carrying, but its additive chemistry can corrode brass and bronze synchronizers in some manual transmissions. Use GL-4 (or a verified yellow-metal-safe / dual-rated oil) where GL-4 is specified.
Is GL-5 the same viscosity as GL-4?
No. GL-4 and GL-5 are performance and EP ratings, not viscosity grades. Either can come in SAE 75W-90, 80W-90, or 85W-140.
What does the SAE number on gear oil mean?
The SAE grade (for example, 75W-90) is the viscosity. The GL rating (GL-4 or GL-5) is the load-carrying and EP performance level. You need both to match your application.