GL-4 and GL-5 are API service classes for gear oils. They tell you how much extreme pressure protection the oil gives.
GL-5: maximum protection for hypoid gears
GL-5 oils carry roughly twice the extreme pressure additive of GL-4. They are designed for hypoid gears in axles and differentials, where gear teeth slide under very high pressure and shock loads.
GL-4: friendly to synchronisers
Many manual gearboxes use brass or bronze synchronisers. Some older GL-5 formulations can be too aggressive for these parts and make shifting harder over time. That is why many gearbox manufacturers specify GL-4.
Oils that meet both
A GL-4/GL-5 oil like Extreme Gear Oil 80W-90 is formulated to protect hypoid gears while staying safe for synchronisers, so one product can cover more of a workshop’s jobs.
The rule of thumb
Follow the vehicle manual. If it asks for GL-4, use GL-4 or a dual rated oil. If it asks for GL-5, never go down to GL-4.
