API GL Gear Oil Grades Explained
Gear oils carry two separate ratings: an SAE viscosity grade like 75W-90 (how thick), and an API GL service grade like GL-4 or GL-5 (how much extreme-pressure protection). The GL grade is the one people get wrong — and using GL-5 where GL-4 is specified can quietly corrode a gearbox. Here is what GL-4, GL-5 and MT-1 mean, and why more EP additive is not automatically better.
Two ratings: how thick, and how much EP
A gear oil label carries two independent things, and confusing them causes most gear-oil mistakes:
- SAE grade (viscosity) — e.g. 75W-90, 80W-90, 85W-140. This is the gear-oil viscosity scale (SAE J306), separate from the engine-oil SAE J300 scale. It sets film thickness cold and hot.
- API GL grade (performance) — GL-4, GL-5, MT-1. This sets how much extreme-pressure additive protects the gear teeth under load and shock.
A 75W-90 GL-4 and a 75W-90 GL-5 have the same viscosity but very different EP chemistry. The viscosity keeps a film; the GL grade stops the film being wiped off under the sliding, high-pressure contact of gear teeth — especially hypoid gears.
GL-4, GL-5 and MT-1
| Grade | EP level | Made for | Yellow-metal safe? |
|---|---|---|---|
| GL-4 | Moderate (~½ of GL-5) | Manual gearboxes, synchromesh, spiral-bevel | Generally yes |
| GL-5 | High (active S–P) | Hypoid axles, differentials, shock load | Often not — check |
| MT-1 | High + thermal durability | Non-synchronised heavy-truck manual boxes | Formulated for it |
| GL-1 to GL-3 | Low / mild | Light-duty, older manual boxes | Yes |
GL-5 typically carries about twice the active sulphur-phosphorus EP of GL-4, because a hypoid differential combines high load with heavy sliding that would tear a GL-4 film. That extra EP is exactly why GL-5 can be too aggressive for a synchromesh gearbox.
Why GL-5 is not an “upgrade” from GL-4
The single most common gear-oil error is fitting GL-5 where the maker specified GL-4, on the logic that a higher number must be better. Two problems:
- Yellow-metal corrosion. GL-5’s active sulphur-phosphorus EP can chemically attack the brass and bronze of synchroniser rings, etching them and causing crunchy shifts over time. GL-4 uses a milder, less-corrosive EP level for exactly this reason.
- Synchro friction. Synchromesh needs a specific friction between rings; the wrong EP chemistry can make a gearbox baulk or over-slip.
If a manual gearbox calls for GL-4, use GL-4 (or a purpose-built GL-4/GL-5 multi-grade proven yellow-metal safe). If a differential calls for GL-5, GL-4 will not protect it. Match the grade to the hardware — do not assume up.
Putting the viscosity and GL grade together
75W-90 GL-4
A light, wide-temperature synthetic-style viscosity with moderate EP — a typical modern manual-transmission oil where synchro protection matters more than raw EP.
80W-90 GL-5
The classic mineral hypoid axle oil — solid film with high EP for differentials and final drives under load and shock.
85W-140 GL-5
A heavy, high-viscosity GL-5 for hot-running, heavily loaded axles — trucks, off-highway, high ambient temperatures.
75W-90 MT-1
A durable oil for non-synchronised heavy-truck transmissions, adding thermal and oxidation stability for long drains and high heat.
Note the “W” works like engine oil (a cold rating), but the numbers themselves are on the SAE J306 gear scale — an 80W-90 gear oil is not the same viscosity as an SAE 90 engine oil. See the SAE grades guide for how W ratings work.
Formulating a gear oil to grade
We formulate gear oils to the exact SAE viscosity and API GL grade you need — selecting the EP/AW additive system and base-oil blend to pass GL-4, GL-5 or MT-1 while staying yellow-metal safe where required, then verify with copper-strip corrosion, EP and wear tests in the lab. See the full GL-5 gear oil formulation. You own the formula and the IP.
Related guides
API GL gear oil FAQ
What is the difference between GL-4 and GL-5?
Both are API gear-oil service grades that rate extreme-pressure (EP) protection. GL-5 carries roughly twice the active sulphur-phosphorus EP additive of GL-4 and is made for hypoid axles and differentials under high, shocking loads. GL-4 has moderate EP for manual gearboxes and synchromesh. They share the same idea but very different EP chemistry.
Can I use GL-5 instead of GL-4?
Not unless the maker approves it. GL-5’s active EP additives can corrode the brass and bronze synchroniser rings in many manual gearboxes and alter synchro friction, causing poor shifting over time. If GL-4 is specified, use GL-4 or a gear oil specifically proven safe for both — do not assume GL-5 is a safe upgrade.
Is a GL grade the same as an SAE grade?
No. The SAE grade (e.g. 75W-90, on the SAE J306 gear scale) rates viscosity — how thick the oil is. The API GL grade (GL-4, GL-5, MT-1) rates extreme-pressure performance. A gear oil is described by both, and they are chosen independently; a 75W-90 can be GL-4 or GL-5.
What is MT-1 gear oil?
API MT-1 is a service grade for non-synchronised (constant-mesh) manual transmissions in heavy commercial trucks. It adds thermal stability, oxidation resistance and seal compatibility for the high temperatures and long drains those gearboxes see, on top of EP protection.
What does yellow-metal compatibility mean?
Yellow metals are the copper alloys — brass and bronze — used in synchroniser rings and bushings. Aggressive sulphur-phosphorus EP additives (as in GL-5) can chemically corrode them. A yellow-metal-safe gear oil uses a milder or better-buffered EP system, verified by a copper-strip corrosion test, so it protects the gears without attacking the synchros.
Need a gear oil built to GL-4, GL-5 or MT-1?
From synchro-safe GL-4 manual-box oils to heavy-duty GL-5 axle oils — we formulate to the exact SAE and API GL grade, keep it yellow-metal safe, and prove it in the lab. The IP is yours.
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