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What’s the difference between transmission fluid and gear oil?

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The simple answer to what makes transmission fluid and gear oil different lies in the purpose that the two solutions were designed for. For instance, transmission fluid is formulated to work with automatic transmissions, while gear oil is typically engineered to work specifically with manual-type gear boxes. It is worth noting however, that as is often the case with automotive lubrication, there are always exceptions to the rule.

For a comprehensive understanding of these two unique fluids, the properties demanded of them and what makes them different, it is important to explore the key differences between the equipment they serve, transmissions and gearboxes.

While both these pieces of equipment are required to perform the same role of enabling gears to change and control the force, speed, and direction of a vehicle, they achieve their own results using different methods and therefore need different supporting fluids for successful operation.

Essentially, both a transmission and a gearbox perform the same role in that they enable a change of gears to control the force, direction and speed, of travel. However, they achieve their objective in different ways and as a result, require different fluids for support in order to function effectively.

Automatic transmissions

Automatic transmissions work in a system that involves multiple gears switching automatically in answer to the different demands of an engine. Gears within an automatic transmission are usually far smaller and greater in number than those contained in a manual system. For effective operations, transmission fluid must not only lubricate but also transfer power from the vehicle’s oil pump to its clutches, the machinery that is constructed to control gear movement.

Key properties of transmission fluid

Traditional transmission fluid is formulated from crude oil through the process of reshaping hydrocarbons to meet specifications of different equipment and vehicles. However, synthetic transmission fluid is engineered through a series of different chemical reactions. As a result, this type of transmission fluid doesn’t break down as easily or oxidize under high temperatures.

Essentially, transmission fluid is a hydraulic oil and an exceptionally thin one. To lubricate the system’s delicate components, it must have a low viscosity so it can flow freely, transferring power from the vehicle engine to its transmission.

Transmission oil contains detergent to ensure no build-up of debris occurs in channels between parts that might negatively affect operations. It must also have heat-resistant properties as it is required to act as a coolant, transferring heat away from working parts when friction occurs. With a much lower boiling point than gear oil, it typically needs additives to increase its lifespan.

Transmission fluid has also been designed for anti-foaming capabilities, keeping unwanted air out of the lubricant so it doesn’t interfere with force being transferred.

Manual gearboxes

Manual gearbox operations are based on a system that features two individual shafts and gears that work together when the user interacts with a gearstick and clutch; because of this, manual systems generate a lot of heat, force and friction. To answer these issues, gear oil must be capable of making these transitions between gears as smooth and seamless as possible, preventing parts becoming damaged as they shift between positions.

Important qualities of gear fluid

Gear oil is composed of two critical elements – the base oil and carefully selected additives. Additives provide desirable properties while suppressing undesirable attributes of the base oil. The additive package is often referred to as the backbone of the oil’s performance, and this foundation provides both the protection and performance users need.

Gear oil is far thicker than standard motor oil. With a high viscosity, the oil can make sure the connected gears are always lubricated and keep them cushioned against the shock and sudden impact when they interact.

Gear oil must also fight any contamination entering the system, particularly water. The gear oil must have the ability to demulsify, which facilitates the easy removal of water from within the gearbox, protecting it against threats like rust.

As manual gearboxes generate an increased amount of friction, they also create a great deal of heat, so gear oil is engineered to withstand exceptionally high temperatures and safely transfer the heat away from the gearbox. The oil must also be able to operate effectively under extreme pressure, so today’s products offered by manufacturers like Fuchs and Mobil often contain specially formulated additives to deliver improved performance and greater oil stability.

Understanding gear oil and transmission fluid applications

As mentioned earlier, there are exceptions to consider when it comes to the rule of thumb for the fluids. While there are some manual gearboxes that do use transmission fluids instead of a dedicated gear oil, the same can never be said for adding gear oil into your automatic transmission. This is because gear oil is far too viscous for automatic transmission systems and can cause clogging in between the different components. This build-up can reduce operational efficiency at a loss of performance, but can also result in damage over time if the issue is left unchecked.

Best practices for using gear oil and transmission fluids

The golden rule that always applies when deploying any automotive fluids and lubricants is to always use the solution that is recommended by the Original Equipment Manufacturer (OEM) of a specific make and model of vehicle. Users should consult their owner’s manual to find details on what products are safe to put in their system before purchasing fluids and oils and performing vehicle maintenance.


At Oil-Store, we stock transmission fluids and gear oils from top brands, including Mobil, Morris, Shell and Q8. Shop the range today.

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