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Gear Oil vs. Hydraulic Oil: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on January 30, 2024
Gear oil is a high-viscosity lubricant designed for the protection of gears under heavy load, while hydraulic oil is a lower-viscosity fluid used for the smooth operation of hydraulic systems.

Key Differences

Gear oil is formulated to handle the high pressure and shear stress in gear systems, ensuring smooth gear engagement and longevity. Hydraulic oil, in contrast, is designed to efficiently transfer power in hydraulic systems and maintain system health.
In terms of viscosity, gear oils are generally thicker to provide a protective film over gear teeth, while hydraulic oils are thinner for rapid flow in hydraulic systems.
Gear oil often contains additives like sulfur-phosphorus for increased load-carrying ability. Hydraulic oil typically has additives to prevent foaming and maintain fluidity under temperature variations.
The thermal stability of gear oil is crucial for preventing breakdown under high temperatures in gearboxes. Hydraulic oil, however, prioritizes hydrolytic stability and low compressibility for consistent hydraulic operation.
Gear oil is used in various transmissions, differentials, and gearboxes. Hydraulic oil finds its use in hydraulic systems like lifts, presses, and steering systems.

Comparison Chart

Primary Function

Protects and lubricates gears
Transfers power in hydraulic systems


Higher viscosity for gear protection
Lower viscosity for smooth fluid flow


Contains load-carrying additives
Includes anti-foaming and fluidity additives

Thermal Stability

High thermal stability for gear environments
Prioritizes hydrolytic stability

Common Uses

Used in transmissions, differentials, gearboxes
Used in lifts, presses, hydraulic steering

Gear Oil and Hydraulic Oil Definitions

Gear Oil

Gear oil often contains sulfur-phosphorus for extra protection.
The gear oil's additives are essential for heavy-duty vehicles.

Hydraulic Oil

Hydraulic oil helps in cooling and lubricating hydraulic components.
Hydraulic oil maintains the health of the hydraulic press system.

Gear Oil

Gear oil is a lubricant specifically designed for transmissions and gearboxes.
The mechanic recommended changing the gear oil every 50,000 miles.

Hydraulic Oil

Hydraulic oil can resist foaming and maintain stability.
Quality hydraulic oil reduces the risk of system failure.

Gear Oil

Gear oil is used in automotive differentials and manual transmissions.
Replacing gear oil improves the performance of manual cars.

Hydraulic Oil

Hydraulic oil is used in hydraulic systems to transfer power.
The hydraulic oil in the excavator ensures smooth movement.

Gear Oil

Gear oil has high viscosity to withstand the stress on gears.
The thick gear oil ensures smooth operation of the heavy machinery.

Hydraulic Oil

Hydraulic oil is characterized by its low viscosity.
The hydraulic oil's fluidity allows for quick response in the machinery.

Gear Oil

Gear oil protects against wear and corrosion in gear assemblies.
Regular gear oil changes keep the gearbox in optimal condition.

Hydraulic Oil

Hydraulic oil is essential in systems like steering and lifts.
The forklift relies on hydraulic oil for lifting heavy loads.


Is gear oil thicker than hydraulic oil?

Yes, gear oil typically has a higher viscosity.

What is the main purpose of gear oil?

Gear oil is primarily used for lubricating and protecting gears.

How often should gear oil be changed?

It varies, but generally every 30,000 to 60,000 miles for vehicles.

What systems typically use hydraulic oil?

Hydraulic oil is used in systems like hydraulic lifts and steering.

Why is low viscosity important for hydraulic oil?

It allows for rapid and efficient power transfer in hydraulic systems.

Can gear oil be used in automatic transmissions?

No, automatic transmissions require specific types of fluid.

How does temperature affect hydraulic oil performance?

Extreme temperatures can affect its viscosity and efficiency.

Can hydraulic oil degrade over time?

Yes, it can degrade and should be periodically checked and replaced.

Can hydraulic oil be used as gear oil?

Not recommended, as they are formulated for different purposes.

Is gear oil harmful to the environment?

Like most oils, it can be if not disposed of properly.

What role does hydraulic oil play in cooling?

It helps in cooling hydraulic systems by dissipating heat.

What additives are found in hydraulic oil?

Common additives include anti-wear, anti-foam, and oxidation inhibitors.

Can gear oil affect fuel efficiency?

Properly maintained gear oil can improve overall vehicle efficiency.

How is hydraulic oil's performance tested?

It's tested for properties like viscosity, corrosion prevention, and thermal stability.

Are there different types of gear oil?

Yes, there are various types for different applications and conditions.

Does gear oil have a smell?

It often has a distinct sulfuric or chemical odor.

How does hydraulic oil affect system response?

Proper hydraulic oil ensures smooth and responsive system operation.

What are the risks of using the wrong gear oil?

It can lead to increased wear, corrosion, and gearbox failure.

Is hydraulic oil flammable?

It can be, especially if it's petroleum-based.

Can gear oil additives extend gear life?

Yes, they can significantly reduce wear and extend gear life.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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