Gear oil protects some of the most heavily loaded components in a vehicle. Differentials, manual transmissions and other geared driveline assemblies can subject lubricant to high pressure, sliding contact, shock loading and elevated temperatures.
Choosing the right gear oil involves more than finding 75W-90, 80W-90 or another familiar viscosity on the bottle.
Several classification systems describe different properties of a gear lubricant. SAE viscosity grades describe how the oil flows at specified temperatures. API GL classifications identify performance levels for different types of gear service. Other specifications, including API MT-1 and SAE J2360, establish additional performance requirements.
The most important distinction is simple:
SAE grade describes viscosity. API GL classification describes performance requirements.
Understanding that difference makes gear oil considerably easier to select.
Table of Contents
- What Is Gear Oil?
- What Does Gear Oil Do?
- How Gear Oil Is Classified
- Gear Oil Viscosity Grades
- What Does 75W-90 Mean?
- Why Are Gear Oil Numbers So High?
- API Gear Oil Ratings
- GL-4 vs. GL-5 Gear Oil
- What Is API MT-1?
- What Is SAE J2360?
- Gear Oil for Differentials
- Gear Oil for Manual Transmissions
- Can Gear Oils Be Substituted?
- How to Choose the Correct Gear Oil
- Gear Oil FAQ
What Is Gear Oil?
Gear oil is a lubricant formulated to protect gears, bearings and other components operating under concentrated loads.
Gear teeth can place tremendous pressure on relatively small contact areas. Some gear designs also create substantial sliding action as the teeth move across one another.
Hypoid gears are a good example. They are widely used in automotive differentials and combine high tooth loading with significant sliding contact.
Under these conditions, an inadequate lubricant can allow increased friction, wear, scoring and eventually component damage.
Gear oils therefore combine suitable base oils with additive systems designed for the operating environment.
The exact formulation depends on the application. A synchronized manual transmission does not necessarily have the same lubrication requirements as a hypoid rear differential, even if both components contain gears.
What Does Gear Oil Do?
The primary job of gear oil is to maintain a protective lubricating film between moving surfaces.
That film reduces friction and helps prevent direct metal-to-metal contact.
Gear lubricant may also need to:
- resist wear and scoring;
- withstand high gear-tooth loads;
- handle sliding contact;
- resist oxidation and thermal breakdown;
- control rust and corrosion;
- maintain viscosity at operating temperature;
- remain sufficiently fluid in cold conditions;
- protect seals; and
- remain compatible with metals used within the assembly.
Modern drivetrains can place substantial demands on these properties. High-performance cars, towing vehicles, commercial trucks and other heavily loaded equipment can generate considerable heat and pressure at the gear surfaces.
How Gear Oil Is Classified
A gear-oil bottle can display several specifications at the same time because each classification can describe a different characteristic.
For example:
SAE 75W-90
This is a viscosity grade.
API GL-5
This is a gear-lubricant performance classification.
API MT-1
This identifies performance requirements for lubricants intended for certain non-synchronized heavy-duty manual transmissions.
SAE J2360
This establishes broader performance requirements for automotive gear lubricants.
A lubricant can therefore be described as SAE 75W-90, API GL-5 and SAE J2360 without those designations being redundant.
They are telling you different things about the lubricant.
This also explains why two bottles marked 75W-90 are not automatically interchangeable. They may have similar viscosity characteristics while meeting different performance requirements.
Gear Oil Viscosity Grades
Automotive gear-oil viscosity is classified under SAE J306.
Common multigrade gear-oil viscosities include:
| SAE Gear Oil Grade | General Description |
|---|---|
| 75W-90 | Broad-range multigrade with good low-temperature fluidity |
| 80W-90 | Common multigrade gear-oil viscosity |
| 75W-140 | Wide-range multigrade used where a heavier operating-temperature viscosity is required |
| 85W-140 | Heavier multigrade gear-oil viscosity |
The grade tells you about viscosity.
It does not, by itself, tell you whether the oil provides the correct performance characteristics for a particular transmission or differential.
What Does the W Mean?
The W indicates a winter viscosity grade.
It does not mean “weight.”
In a multigrade such as 75W-90, the 75W portion relates to low-temperature performance, while the 90 portion represents the lubricant’s higher-temperature viscosity classification.
A multigrade gear oil can therefore provide useful low-temperature fluidity while maintaining the required viscosity as operating temperature rises.
What Does 75W-90 Gear Oil Mean?
75W-90 is an SAE multigrade gear-oil viscosity classification.
The 75W portion identifies low-temperature viscosity performance. The 90 identifies its higher-temperature viscosity grade.
This does not mean the lubricant physically changes from an SAE 75 oil into an SAE 90 oil as it warms.
Instead, a 75W-90 satisfies the applicable viscosity requirements for both portions of the multigrade classification.
However, knowing that an oil is 75W-90 still does not tell you whether it is suitable for your differential or transmission.
You must also check its required performance specification.
Why Are Gear Oil Numbers So High?
This causes a lot of confusion.
An SAE 75W-90 gear oil is not necessarily dramatically thicker than SAE 10W-40 motor oil simply because 90 is a much larger number than 40.
Motor oil and automotive gear oil use different SAE viscosity grading systems.
The numbers therefore cannot be directly compared.
Viscosity ranges on the two scales can overlap. For example, a 75W-90 gear oil can have operating-temperature viscosity similar to a 10W-40 motor oil.
The viscosity comparison chart in the August 2026 technical source illustrates this overlap among SAE crankcase oil, SAE gear oil, ISO and AGMA viscosity classifications.
This leads to an important rule:
Never compare motor-oil and gear-oil thickness solely by the SAE numbers printed on the containers.
A larger SAE gear-oil number does not automatically mean the lubricant is proportionally thicker than a motor oil carrying a smaller number.
API Gear Oil Ratings
API gear-lubricant classifications use the designation GL, meaning Gear Lubricant.
These classifications address the lubricant’s performance under different levels of gear loading, sliding contact and extreme-pressure service.
API GL-1
GL-1 represents relatively light-duty gear service.
Lubricants in this category traditionally contain little or no extreme-pressure additive treatment.
That makes GL-1 fundamentally different from the higher-performance gear oils required for heavily loaded hypoid gears.
GL-1 should never be treated as a generic substitute where GL-4 or GL-5 is specified.
API GL-4
GL-4 provides performance for applications requiring moderate extreme-pressure protection.
It is commonly associated with manual transmissions and transaxles.
These applications present an interesting lubrication challenge. The lubricant must protect gears while also working correctly with synchronizers and the materials used in their construction.
Copper-containing alloys are commonly used in synchronizer components, making material compatibility an important part of lubricant selection.
API GL-5
GL-5 addresses severe gear service.
The classic application is the hypoid automotive differential.
Hypoid gear sets experience substantial sliding contact along with high torque loads. These operating conditions require strong extreme-pressure protection to help prevent damaging metal-to-metal contact.
The API classification therefore tells us something fundamentally different from the SAE viscosity grade.
GL-4 vs. GL-5 Gear Oil
A common mistake is assuming:
GL-5 must be better than GL-4 because 5 is higher than 4.
That is not the right way to choose gear oil.
GL-4 and GL-5 address different levels and types of gear service.
| Characteristic | API GL-4 | API GL-5 |
|---|---|---|
| Extreme-pressure requirements | Moderate | Severe |
| Common association | Manual transmissions and transaxles | Hypoid differentials |
| Sliding/load severity | Moderate | High |
| Synchronizer considerations | Often important | Application dependent |
| Automatically interchangeable | No | No |
GL-5 provides more severe extreme-pressure performance, but that does not automatically make it the correct lubricant for a transmission designed around GL-4 requirements.
The proper specification comes from the component manufacturer.
Higher GL number does not automatically mean better for your application.
What Is API MT-1?
API MT-1 addresses lubricants intended for non-synchronized manual transmissions used in heavy-duty service.
These applications have requirements that extend beyond basic gear protection.
MT-1 includes performance considerations involving:
- thermal stability;
- cleanliness;
- seal compatibility; and
- compatibility with copper-containing alloys.
MT-1 is another good example of why viscosity alone cannot identify the correct lubricant.
A heavy-duty transmission might require a particular SAE viscosity while also requiring MT-1 performance.
Both requirements matter.
What Is SAE J2360?
SAE J2360 is a performance specification for automotive gear lubricants.
Its roots are connected to military gear-lubricant requirements. The older MIL-PRF-2105 specification has largely been superseded by SAE J2360, which harmonizes military and commercial gear-lubricant performance requirements.
SAE J2360 addresses more than viscosity.
Performance considerations include:
- load-carrying capability;
- oxidation resistance;
- corrosion protection;
- seal compatibility; and
- compatibility with metals, including copper-containing alloys.
An SAE viscosity grade tells you how a lubricant behaves from a viscosity standpoint.
SAE J2360 tells you considerably more about whether the complete lubricant formulation can satisfy defined performance requirements.
Gear Oil for Differentials
Differentials are among the most familiar automotive gear-oil applications.
Many automotive differentials use hypoid gears.
Unlike simple parallel-axis gears, a hypoid arrangement positions the pinion away from the ring gear’s centerline. This geometry provides useful strength and packaging characteristics but produces considerable sliding action between the gear teeth.
That sliding contact creates demanding lubrication conditions.
Consequently, GL-5 lubricants are commonly associated with automotive hypoid differentials.
However, GL-5 alone does not identify the correct oil.
A differential may require:
- 75W-90;
- 80W-90;
- 75W-140;
- another viscosity;
- a specific OEM specification; or
- additional limited-slip characteristics.
Vehicle load, axle design, operating temperature and duty cycle can all influence the manufacturer’s requirement.
Gear Oil for Manual Transmissions
Manual transmissions can make gear-lubricant selection more complicated.
Protecting the gear teeth is only one part of the job.
A synchronized manual transmission uses synchronizers to help match rotational speeds during shifting. The lubricant therefore needs appropriate frictional characteristics in addition to adequate gear protection.
Material compatibility can also matter because synchronizer assemblies may contain copper alloys.
This helps explain why a GL-5 differential lubricant should not automatically replace a GL-4 manual-transmission lubricant.
More extreme-pressure capability does not necessarily equal better transmission performance.
Heavy-duty non-synchronized transmissions present another set of requirements and may specify API MT-1 or other manufacturer-specific standards.
Use the specification required for the transmission, not simply the highest GL rating available.
Can You Substitute One Gear Oil for Another?
Sometimes, but matching the viscosity is not enough.
Suppose two bottles both say:
SAE 75W-90
One may be intended primarily for GL-5 differential service, while another may carry additional specifications or approvals.
Before substituting gear oils, compare:
- SAE viscosity grade
- API GL or MT classification
- OEM specification
- Transmission or differential design
- Limited-slip requirements
- Operating temperature and duty
If the required performance specification does not match, identical viscosity numbers do not make the lubricants equivalent.
This is particularly important with manual transmissions, synchronized gearboxes and limited-slip differentials.
How to Choose the Correct Gear Oil
Start with the vehicle or equipment manufacturer’s lubrication specification.
Identify four things before selecting a product.
1. Required Viscosity
Examples include:
- 75W-90
- 80W-90
- 75W-140
- 85W-140
2. Required Performance Classification
Examples include:
- API GL-4
- API GL-5
- API MT-1
3. Required Industry or OEM Specification
The application may require SAE J2360 or a manufacturer-specific specification.
4. Special Requirements
Some applications have additional considerations, such as limited-slip differentials, synchronizers or specific material-compatibility requirements.
Once these requirements are known, compare lubricants that satisfy them.
Do not work backward by choosing a brand, synthetic versus conventional base oil or the “highest” GL number first.
Specification first. Product second.
That approach dramatically reduces the chances of putting the wrong lubricant into a transmission or differential.
Gear Oil FAQ
Is 75W-90 thicker than 10W-40?
Not necessarily. SAE gear oil and SAE motor oil use different viscosity grading systems, so their grade numbers cannot be directly compared. Their viscosity ranges can overlap, and 75W-90 gear oil can have operating-temperature viscosity similar to 10W-40 motor oil.
What is the difference between GL-4 and GL-5?
GL-4 addresses moderate extreme-pressure gear service and is commonly associated with manual transmissions and transaxles. GL-5 addresses more severe service and is commonly associated with hypoid differentials.
Does GL-5 mean better than GL-4?
No. GL-5 represents a more severe extreme-pressure performance level, but that does not make it correct for every application. Use the GL classification specified by the equipment manufacturer.
What does 75W-90 mean?
75W-90 is a multigrade SAE gear-oil viscosity classification. The 75W portion describes low-temperature viscosity performance, while the 90 represents its higher-temperature viscosity grade.
What does the W in 75W-90 mean?
The W identifies a winter viscosity classification. It does not mean “weight.”
Is gear oil the same as differential oil?
Differential oil is a type of gear lubricant, but not every gear oil is appropriate for every differential. The viscosity and performance specifications must match the differential requirements.
Can I use GL-5 instead of GL-4?
Do not substitute GL-5 for GL-4 solely because GL-5 has a higher number. Manual transmissions and synchronized gearboxes can have specific frictional, additive and material-compatibility requirements. Follow the manufacturer’s specification.
Is SAE J306 the same as API GL-5?
No. SAE J306 defines automotive gear-oil viscosity grades. API GL-5 is a gear-lubricant performance classification. A lubricant can therefore be both SAE 75W-90 and API GL-5.
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