Gearbox oil or transmission fluid?

The first question we’re going to cover before we get anywhere near debating what fluid or oil changes should or shouldn’t cost, is which one you actually need.

Gearbox oil? Transmission fluid? It’s two ways of saying the same thing, right?


There is a difference that can make or break your car

Transmissions, or gearboxes, can be manual or automatic. The type of fluid or oil each requires is unique, as they perform related, yet quite different tasks.

Using the wrong type of fluid or oil can lead to excessive wear and tear. In turn, that wear will reduce the life of your gearbox or transmission, and lead to early replacement and unnecessary expense.

Automatic transmission fluid (ATF)

Automatic transmission fluid is a complex fluid.

It contains not only the lubricants and detergents to keep the transmission running smoothly, but also operates as a hydraulic fluid for your transmission. In some automatic cars, it also acts as the hydraulic fluid for the power steering and 4-wheel-drive arrangement.

Because of the nature of its work within your automatic transmission, the ATF also needs to incorporate anti-wear additives to combat rust and corrosion. It also needs to be finder due to the cogs and gears are much smaller than in a manual gearbox.

And that’s not all.

Automatic transmission fluid carries many additional additives and extras compared to traditional engine oil. If you really want to know how many, and how much more work it carries out, here’s your list:

  • Dispersants and surfactants to protect and clean metal components
  • Viscosity improvers
  • Seal swell additives
  • Anti-foam additives
  • Anti-oxidant compounds
  • Cold-flow improvers
  • High-temperature thickeners
  • Gasket conditioners
  • Pour point depressants
  • And finally, some form of petroleum dye.

Manual gearbox oil

Gearbox oil lubricates your manual transmission while also protecting it from corrosion.

Due to the nature of the high friction between large moving components during manual gearbox changes, the oil carries anti-wear compounds. They generally feature sulphur, which gives them a strong and distinctive smell. Gearbox oil is a much thicker and heavier duty beast all round.

How to spot the difference between automatic transmission fluid and gearbox oil at a glance

Automatic transmission fluid is much thinner than gearbox oil, and because of the dye it contains, is usually red or green.

The easiest way to find out which it is is to read what it says on the label. If the oil you’re considering using doesn’t have a label, then do yourself a big favour and get rid of it straight away.

Any unknown oil or fluid has no place in your car’s engine or components. Get it recycled or dispose of it properly. Otherwise, you could well be poisoning your car.

Automatic transmission fluid replacement prices

A typical drain and refill of automatic transmission fluid could cost you anywhere around £150 to £200.

For performance cars, you’ll probably pay a little more, as you would expect. The prices will be dependent on the quality of fluid and parts, and any specialised requirements for your make and model.

When should you change your transmission fluid?

A change and refill—or transmission fluid flush— should happen at around 40k mile intervals.

If your car regularly pulls heavy loads (caravans or trailers), you might want to consider slightly shorter periods.

Whenever you change your ATF, you should also change the filter and pan gasket. Not all automatic transmissions have a filter, so don’t panic if you can’t find one.

A healthy ATF should appear almost clear and have a slightly sweet smell. If you apply a single drop to a paper towel, it should spread out to around a 3cm width, with no discolouring or spots.

Symptoms of poorly operating transmission fluid

  • Your engine overheats
  • The vehicle struggles to run comfortably
  • When your vehicle stalls when driving up or down steep inclines
  • When your check engine light is illuminated
  • If the transmission makes unexpected noises or feels lumpy in its performance

Overheating causes the majority of automatic transmission failures, and is often brought about by old transmission fluid that should have already been replaced.

Why your automatic transmission could fail and why fluid replacement is so important

Hydraulic issues

As already mentioned, your automatic transmission isn’t solely a cog-selecting operation. It operates by driving pressurised fluid through its components.

To do this smoothly and efficiently, and through the maze of filters, metering valves and tiny orifices it has to, it needs to stay as clean and as pure as possible.

Obviously, that’s easy said than done with components that are in constant connection with each other. Everyday operation will cause general wear, leaving tiny fragments and debris floating in your transmission fluid.

Debris in your transmission fluid

An automatic transmission operates using several clutches, which will all suffer wear over time. The nature of a clutch slipping also dictates the constant rubbing that delivers tiny particles of itself into the fluid used to lubricate them.

This debris will eventually affect the transmission’s smooth operation. If you don’t replace the fluid it’s floating around in, the additional wear will accelerate the erosion of your transmission’s working parts.

Transmission blockages

When the debris is allowed to build up, it impedes the operation of the transmission components. Sticking components will cause a harsh or delayed gear change. A blockage will prevent them entirely.

The worst outcome is a blockage in the main filter. A complete blockage is usually the result of too much debris. If no fluid can pass through the transmission, then this will prevent its operation entirely.

Partial blockages

An only partially blocked transmission filter will starve the transmission of the fluid it needs to operate efficiently.

Insufficient fluid supply will cause clutches to lose power and performance, and result in poor driving operation. As outlined previously, a lack of fluid will encourage excess wear, introducing additional debris, whereas a total blockage will cause your transmission to stop working entirely.

The difference between the two could mean a simple fix of a new filter and fluid change, or a much more expensive one, requiring an overhaul or a brand new transmission.

Lifetime transmission fluids and sealed units

Some manufacturers produce cars with sealed transmission units and claim that the oil or fluid they use will never need changing.

The problem with sealed units is that the estimated lifetime of the transmission unit is longer than that of the fluid. In effect, the transmission will fail before the fluid requiring its replacement.

If your sealed gearbox does die on you, the fact that you never had to pay for a fluid change will be little consolation when faced with the cost of replacing the entire unit. We’d still advise that a mechanic checks the quality of the fluids to determine whether or not there are any visible signs of trouble.

Fault finding, especially at your car’s regular service, can always stand to save you from those expensive problems you might have easily have avoided.

About Fixter

Fixter is revolutionising the car maintenance industry, one repair at a time. Fixter was founded to make car maintenance as easy as booking a taxi. Digital, transparent and stress-free, with world-class customer service. Since launching in Manchester in 2017, Fixter has expanded to more than 100 cities across the UK and provided thousands of car owners with honest, convenient and affordable car repair services.