Key summary about gear oil
- Gear oil contains specialised additives that help protect against rust, corrosion and wear.
- Manual transmissions, differentials and transfer cases may all require gear oil for lubrication.
- Using the wrong gear oil can affect performance and increase component wear over time.
Gear oil reduces metal-to-metal contact.
Vehicles require different gear oil grades.
Gear oil helps reduce noise and vibration.
Gear oil is the broad term for several different types of lubricants and fluids that support the functioning and performance of the gear systems within a vehicle. Similar to engine oil (although not the same), it serves multiple purposes, from protecting gears from wear and tear to dissipating heat and reducing friction.
An essential ingredient to the smooth running of your car, here we’ll go over five things you need to know about gear oil, and cover some FAQs about this crucial lubricant.
1. There are different types of gear oil
Gear oil is a vague term that could refer to multiple fluids used in various vehicle types.
Manual transmission fluid
Most commonly, gear oil refers to manual transmission fluid (MTF), a lubricant that reduces friction inside a manual gear system.
Automatic transmission fluid
On the other hand, automatic transmission fluid (ATF) is used in automatic transmissions where gear shifts happen without manual intervention, supporting the transfer of power from the transmission to the engine. It has slightly different properties from MTF because it operates under less heat and pressure.
Other types of lubricants include differential gear oil and transfer case fluid.
Differential gear oil
Every vehicle has a differential that allows the wheels to rotate at different speeds when turning. Differential gear oil is a lubricant used to lubricate the gears and bearings inside the differential. Its unique additive formulation helps reduce wear and tear on these components that experience high pressures and shock loads during acceleration, braking, and cornering.
Transfer case fluid
In four-wheel drive vehicles, there is also a transfer case that directs power from the transmission to all four wheels. Transfer case fluid lubricates the gears and bearings inside this transfer case. While not under the same extreme conditions as the differential, the transfer case fluid still contains protective additives to ensure proper lubrication and prevent premature wear.
Using the correct differential and transfer case fluids recommended by the manufacturer is important for the long-lasting performance of these drivetrain components.
2. Why viscosity is important
Different types of gear oil have different viscosity (thickness) grades. Gear oil viscosity is usually graded by two numbers separated by a ‘W’, such as 75W-90. The ‘W’ stands for winter, so the first number indicates the oil’s viscosity or property at a cold temperature. The second number indicates the oil's viscosity/property at higher temperatures.
The higher the numbers, the greater the viscosity (thickness) of the oil. Thicker gear oils are typically used for higher heat and high-pressure applications, and mostly in manual transmission vehicles.
Automatic transmission fluid tends to be much less viscous because it needs to flow freely for smooth transmission of power from the engine to the wheels.
3. Synthetic vs. conventional gear oil
When choosing gear oil, it's important to understand the differences between synthetic and conventional formulations and how they can affect vehicle performance.
Synthetic gear oil
Synthetic gear oil is gaining popularity as a high-performance alternative to traditional petroleum-based lubricants. Synthetic gear oil can typically handle higher temperatures and can be fine-tuned at a more molecular level to enhance specific performance metrics.
Viscosity remains more constant as temperature changes, making it more reliable in fluctuating conditions. Finally, synthetic gear oil tends to form a stronger film to protect internal components, which lasts longer than its traditional mineral counterpart.
Conventional gear oil
Conventional (mineral) gear oil is a petroleum-based lubricant that remains a trusted choice for many vehicles. It provides dependable lubrication and protection for gears and transmission components when maintained correctly.
It is often a more cost-effective option than synthetic gear oil and may be recommended for certain older vehicles or applications where synthetic formulations are not required.
Having said all of this, quality mineral gear oil from a trusted brand will still offer a range of benefits to help you shift gears smoothly.
Frequently asked quetions
Understanding the importance of gear oil
Choosing the right gear oil and maintaining it correctly can help protect your vehicle's transmission and support long-term performance.
- Use the gear oil grade and specification recommended by your vehicle manufacturer.
- Check and replace gear oil according to the recommended service schedule.
- Watch for signs of worn or contaminated gear oil, such as unusual noises or poor transmission performance.
- Address gearbox issues early to help prevent unnecessary wear and costly repairs.
- Regular maintenance can help keep your transmission operating smoothly and efficiently.
For more vehicle maintenance advice and expert insights, explore our other helpful blog posts.
Explore our car fluids
Sources for further reading
Engine & Gear Oil Explained: Their Purpose & Grading, Demon Tweeks
How to maintain and repair electric vehicles, The AA
Transmission Oil, Euro Car Parts
Should Synthetics be Mixed with Mineral Oils?, Machinery Lubrication