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Gear (transmission) oils: roles, types and applications

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The gear systems in your car (transmission) engage in a lot of metal-on-metal contact. The role of the gear (transmission) oil is to lubricate and protect the gear system, facilitate the functioning of your car’s transmission and secure a comfortable drive.

Poor lubrication of this system, on the other hand, can result in corrosion, scuffing, and pitting of important drivetrain components. This deterioration can compromise your car's performance, and eventually lead to costly repairs. Hence it is essential that you use the right gear oil, perform regular check-ups and change it in accordance with the manufacturer’s recommendations.

In essence, there are two types of gear oils. These are fluids for manual transmission (MTF) and differentials and automatic transmission fluids (ATF). Specifications may vary depending on the car model, so it is crucial to rely on the manufacturer’s recommendation or advice given by an authorized service or experts in this field.

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Manual transmission and differential gear oils

The gear and differential system provide optimal synchronization between the engine revolutions and wheel rotation. The differential enables the wheels to turn at different speeds when turning.

When it comes to the change interval, manual transmission systems usually do not require oil change, seeing it as most manufacturers have switched to the “fill-for-life” system. However, this is not always the case. Some Japanese manufacturers recommend that the gear oil change interval be 7 or 8 years. When it comes to certain models of Honda and Chevrolet, the interval is every 2 or 3 years. By all means, consult the owner’s manual for the recommended change interval.

What is even more important is to check the level of gear oil according to the manufacturer’s instructions, and top up if needed. Topping up usually requires special tools so it is best to leave this procedure to your mechanic.

In any case, gear oil should be changed when having your gear system repaired or clutch system replaced.

Automatic transmission fluid

Automatic transmission fluids are also known by the abbreviation ATF. An ATF also performs the function of fluids used in complex hydraulic systems, which enable smooth gear shifting.
In essence, automatic transmission fluids are used for lubricating automatic transmissions, power steering systems and as torque converters.

The change interval for an automatic transmission fluid is recommended by the vehicle manufacturer and it is usually every 60,000 km to 120,000 km, although there are exceptions.

Some cars, predominantly older models, allow checking ATF level using a dipstick. Newer automatic transmission systems, however, most often do not have such a dipstick so every check and change should be done in a service.

If you are buying a used car, make sure you test the automatic transmission in both cold and warm mode of operation. Ask the seller when the fluid was last changed. If they can’t provide this information, consult your car service.

Probna vožnja

ATF that is used for power steering systems is usually Dexron II type, however, there are also special high viscosity hydraulic fluids for that purpose. In any case, follow the manufacturer’s recommendation. If the instruction manual tells you that this fluid should be changed, by all means do that. If not, maintain the fluid level and if you experience any kind of difficulty in using the gear stick, or hear strange sounds, go to a service center where they will either change the fluid, or do a necessary repair.

What happens if I don’t change gear oil in time?

If you don't change your gear oil according to the manufacturer's suggested schedule, you risk severe damage to your car's transmission and other gear systems. This could mean having to replace parts and repair your transmission, at substantial cost.
Transmissions and other gear systems function with a lot of metal-on-metal contact, natural friction grinds these moving parts down, and tiny metal particles slowly accumulate in the gear oil. These factors eventually compromise gear oil's lubricating qualities, so if your gear oil is not drained and replaced periodically, it will shorten the life of your transmission. 

What are signs that gear oil should be changed?

Pay attention to how your transmission feels as you shift gears. Does it slip or thump? Strange burning smells or whining or grinding sounds are also signs you should change your gear oil. Look out for leaks, as they can indicate contaminated oil. If your gear oil is dark or gritty, it's ready for a change. Signal lights on your dashboard can also warn you about the gear oil overheating, and some other issues with the transmission system.
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