GL-4 vs GL-5 gear oil: what is the difference?
Gear oil works under extreme pressure between gear teeth. The API GL rating tells you which pressure level the oil is built for.
What GL-4 and GL-5 mean
Both contain extreme-pressure (EP) additives that protect tooth surfaces under heavy load. GL-5 has roughly twice the EP additive level of GL-4 and is made for hypoid differentials and heavy loads. GL-4 is mainly for manual gearboxes with synchronisers.
Why not always use GL-5?
The sulphur-phosphorus additives in GL-5 can attack brass and bronze synchronisers in some gearboxes or change their friction, making shifts harder. If your manual specifies GL-4, do not use a plain GL-5 — unless the oil is dual-rated GL-4/GL-5.
Gear oil grades
- 75W-90: synthetic or semi-synthetic; smooth cold-weather shifting.
- 80W-90: the most common grade for manual gearboxes and differentials.
- 85W-140: for truck differentials and heavily loaded axles in hot climates.
Note that gear oil numbers are not comparable with engine oil numbers: an 80W-90 gear oil is roughly as thick as a 15W-40 engine oil.
When to change it
Many passenger-car manual gearboxes keep their oil for 60,000 km or more, but trucks, heavy work, dust or water crossings need shorter intervals. Your manual is the reference.
Quick answers
Can I use GL-5 where GL-4 is specified?
What does a differential need?
Is automatic transmission fluid the same as gear oil?
See our gear & transmission oils range, or ask our sales team for a recommendation and price.
