GL-4 vs GL-5 Gear Oil: What's the Difference?
GL-4 and GL-5 are the two API gear-oil service ratings you will see most often, and they are frequently — and wrongly — treated as interchangeable. The difference is the level of extreme-pressure (EP) additive, and getting it wrong can either leave gears unprotected or quietly corrode your gearbox synchronisers. Here is what each rating means and how to choose.
What the GL ratings mean
API GL ratings describe the load-carrying and EP performance of an automotive gear oil. GL-4 and GL-5 use the same base test but differ in additive concentration: GL-5 carries roughly double the EP additive of GL-4, for higher-load applications like hypoid differentials.
GL-4 — for manual gearboxes and synchros
GL-4 oils have a moderate EP additive level suited to manual transmissions, transaxles and synchromesh gearboxes. The lower active-sulphur content is gentler on the 'yellow metals' (brass and bronze) used in synchroniser rings, which is why most manual gearboxes specify GL-4.
GL-5 — for hypoid axles and differentials
GL-5 oils carry a higher EP additive dose for the intense sliding loads in hypoid final drives, differentials and heavy axles. That protection comes from more active sulphur-phosphorus chemistry, which can be corrosive to yellow metals if used where it is not required.
The yellow-metal / synchro issue
This is the heart of the matter. Using a GL-5 oil in a gearbox that specifies GL-4 can, over time, attack brass and bronze synchroniser components, leading to harder shifting and wear. Conversely, using GL-4 in a hypoid differential that needs GL-5 leaves the gears under-protected against scoring. Always follow the equipment maker's specification.
GL-4 vs GL-5 at a glance
| Property | GL-4 | GL-5 |
|---|---|---|
| EP additive level | Moderate | High (≈2× GL-4) |
| Yellow-metal safe | Generally yes | Not always |
| Typical use | Manual gearboxes, synchros | Hypoid axles, differentials |
| Load capacity | High | Very high |
| Use interchangeably? | No — follow OEM spec | No — follow OEM spec |
So can you use GL-5 instead of GL-4?
Not by default. If the gearbox calls for GL-4, use GL-4 (or a verified yellow-metal-safe GL-4/GL-5 dual-rated oil). If the axle calls for GL-5, use GL-5. The grade (for example EP 90 or EP 140) sets the viscosity; the GL rating sets the additive chemistry — match both to the specification.