Gear Oil Explained: Types, Viscosity & FAQs
28 Mar 2024
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. Most commonly, gear oil refers to manual transmission fluid (MTF), a lubricant that reduces friction inside a manual gear system.
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 to MTF because it operates under less heat and pressure.
Other types of lubricants include differential gear oil and transfer case fluid.
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 this 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.
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. 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 temperature.
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
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.
With synthetic, viscosity remains more of a 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.
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.
Gear Oil FAQs
Can I put gear oil in my engine?
Gear oil and engine oil are not interchangeable. Gear oil is specially designed for transmissions, differentials and gear systems, and using it in your engine can lead to serious damage. Be sure to check your vehicle’s handbook and use the appropriate engine oil for your make and model instead.
Can you mix gear oil brands?
For optimal performance, we recommend you follow the manufacturer’s recommendations and only use one brand of gear oil. However, it is possible to mix gear oil brands, providing both fluids have the same viscosity, API rating, and application.
Can you mix mineral and synthetic gear oil?
We don’t recommend mixing different types of gear oil. Blending mineral and synthetic gear oil could result in unpredictable performance and potentially lead to damage.
What gear oil do I need?
To find out what type of gear oil you need, you need to check your vehicle’s manual or look up your manufacturer’s recommendations online. Be sure to buy the specific fluid type and viscosity grade for your vehicle, taking into account your local climate and driving habits.
Do EV vehicles need oil?
Fully electric vehicles (EVs) do not need engine oil, as they do not have a traditional internal combustion engine. This is a major benefit of EV ownership, as it eliminates the most common and frequent maintenance task associated with gasoline cars. They do, however, require other EV fluids.