Walk into any workshop and you'll hear it: "Just use GL-5, it's the better oil." It's one of the most common misconceptions in automotive lubrication, and it can cost you a gearbox.
GL-4 and GL-5 aren't a "good" and a "better" version of the same thing. They're designed for different jobs. Understanding the difference means understanding what the GL rating measures, what the additives do, and how they interact with the metals inside your transmission.
This guide covers what you need to know to choose the correct automotive gear oil for your equipment.
What Does the "GL" Rating Mean?
The GL (Gear Lubricant) classification was developed by the American Petroleum Institute (API) to describe a gear oil's performance level, specifically the severity of the service conditions it's designed for.
The classifications run from GL-1 to GL-6, but only two are widely used today:
- API GL-4: for moderate conditions, typically manual transmissions and transaxles, plus spiral-bevel and hypoid gears in less severe service.
- API GL-5: for severe conditions, typically hypoid gears in differentials and axles under high speed, heavy load and shock loading.
The GL rating does not tell you the oil's thickness. That's a separate measurement, covered below.
The Real Difference: Extreme Pressure (EP) Additives
The key difference between GL-4 and GL-5 is the amount of extreme pressure (EP) additive in the oil.
In heavily loaded gears, the oil film between the gear teeth can be squeezed so thin that metal touches metal. EP additives, usually sulphur-phosphorus compounds, react with the metal surface under heat and pressure. They form a protective layer that prevents scuffing, scoring and welding of the gear teeth.
- GL-4 oils contain a moderate level of EP additive, typically around half the level found in a GL-5 oil.
- GL-5 oils contain a high level of EP additive, designed to protect gears under the most extreme sliding and loading conditions.
More EP additive sounds like more protection, and for the right gears it is. But those additives are chemically active, and that's where the "GL-5 is always better" idea falls apart.
Why Gear Type Matters
Different gear designs put very different demands on the oil.
Hypoid gears, found in most rear-wheel-drive differentials and truck axles, have offset gear teeth that slide heavily against each other as well as rolling. That sliding action creates intense pressure and heat at the tooth surface. These gears need the high EP protection of a GL-5 oil. Use a GL-4 oil here and you risk scoring, pitting and premature failure.
Manual gearboxes and transaxles mostly use helical and spur gears with far less sliding. The gears themselves don't need GL-5-level EP protection. What these gearboxes do have are components that can be sensitive to it.
Synchronisers: Where GL-5 Can Cause Problems
Most manual gearboxes use synchronisers (synchro rings) to match gear speeds so shifts are smooth. Many of these synchros are made from yellow metals such as brass or bronze, and they rely on a specific level of friction to work properly.
GL-5 oils can affect synchronisers in two ways:
- Friction. High EP additive levels can make the oil too slippery for the synchro rings to grip. The result is notchy or crunchy gear changes, and synchros that wear out prematurely.
- Chemical activity. In some formulations, the active sulphur that protects hypoid gears can, at high temperatures, attack yellow-metal components over time.
That's why a gearbox specified for 75W-90 GL-4 shouldn't simply be filled with a GL-5 oil of the same viscosity. It's also why the reverse is true: using GL-4 in a hypoid differential that calls for 80W-90 GL-5 leaves the gears under-protected.
Modern formulation has blurred the line somewhat. Some oils carry dual GL-4/GL-5 ratings or the heavy-duty API MT-1 rating, using additive chemistry that is safe for synchronisers while still offering high load protection. But a dual rating is a property of that specific product, not a general rule. It must be confirmed on the product data sheet.
GL Rating vs Viscosity: Two Different Things
A common source of confusion is treating the viscosity grade and the GL rating as the same thing. They're not.
- Viscosity (e.g. 75W-90, 80W-90, 85W-140) is defined by the SAE J306 standard. It describes how thick the oil is at low and high temperatures.
- The GL rating describes the oil's performance level and additive content.
Any viscosity grade can come in either performance level. A 75W-90 GL-4 and a 75W-90 GL-5 have similar thickness but very different additive chemistry, and they aren't interchangeable.
| Viscosity grade | Typical use | Common GL rating |
|---|---|---|
| 75W-80 / 75W-90 | Manual gearboxes, transaxles, some modern axles | GL-4, GL-5 or dual-rated. Check the spec. |
| 80W-90 | Differentials, axles, older manual gearboxes | Commonly GL-5, but GL-4 versions exist |
| 85W-140 | Heavily loaded differentials, trucks, high-temperature operation | Typically GL-5 |
This is why matching the viscosity alone isn't enough.
Quick Comparison: GL-4 vs GL-5
| API GL-4 | API GL-5 | |
|---|---|---|
| EP additive level | Moderate | High |
| Designed for | Manual gearboxes, transaxles, moderate-duty gears | Hypoid differentials and axles under severe load |
| Synchroniser compatibility | Generally designed with synchros in mind | May affect synchro friction or yellow metals, unless the product is specifically approved |
| Risk if used in the wrong place | Under-protection of hypoid gears | Poor shift quality and synchro wear in some manual gearboxes |
| "Better"? | Correct for its application | Correct for its application |
Manufacturer Specifications: The Final Word
API ratings are a baseline. Many vehicle and component manufacturers go further and publish their own gear oil specifications, such as ZF, MAN, Mercedes-Benz, Volvo and Scania approvals. These can include:
- Specific additive chemistry and friction properties
- Seal compatibility requirements
- Extended drain interval approvals
- Limited-slip differential friction requirements (often labelled "LS")
- Thermal stability and oxidation limits
A gear oil can meet API GL-5 and still not be approved for a particular OEM specification. For fleets, agricultural equipment and heavy-duty vehicles, running a non-approved oil can affect performance, drain intervals and even warranty cover.
At LubeLine, we supply gear oils that carry these OEM approvals, including the Shell Spirax range for transmissions, axles and drivetrains, alongside products from Fuchs, Valvoline and our other partner brands.
What About Industrial Gearboxes?
GL ratings apply to automotive and mobile equipment gear oils. Enclosed industrial gearboxes in plants, conveyors and mining equipment use a different system, typically ISO viscosity grades (such as ISO VG 220 or 320) and industrial standards like DIN 51517-3 (CLP).
If you're selecting oil for plant equipment, see our range of industrial gear oils or talk to our technical team.
Frequently Asked Questions
Can I use GL-5 instead of GL-4?
Only if the manufacturer permits it, or if the specific GL-5 product is also approved for GL-4 applications. In many manual gearboxes with yellow-metal synchronisers, a standard GL-5 can cause poor shifting and synchro wear.
Can I use GL-4 instead of GL-5?
Not in a hypoid differential or axle that calls for GL-5. GL-4 doesn't contain enough EP additive to protect heavily loaded hypoid gears.
Is 75W-90 GL-4 or GL-5?
It can be either. 75W-90 is a viscosity grade, not a performance level. Always check the GL rating and OEM approvals on the product data sheet.
What does "LS" mean on a gear oil?
LS stands for limited slip. These oils contain friction modifiers for limited-slip differentials. Using a non-LS oil in an LSD can cause chatter and noise.
Should I mix GL-4 and GL-5 oils?
No. As with other lubricants, mixing different formulations can compromise performance. See our FAQ on mixing lubricants.
Need Help Choosing the Right Gear Oil?
Whether you're maintaining a heavy-duty truck fleet, agricultural equipment or a workshop's worth of passenger vehicles, our technical team can match the correct gear oil to your equipment's specification. We can also cross-reference the product you're currently using.
Contact our Johannesburg team or our Cape Town team for product advice and a quote, with 24–48 hour delivery across Gauteng and the Western Cape.
The bottom line: GL-5 isn't better than GL-4. It's different, and each is right for its own application. Always follow the equipment manufacturer's required specification rather than selecting gear oil based solely on viscosity.
