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What Is Transmission Fluid? (And When Do You Need to Replace It?)

Your car's transmission - or gearbox - is a complex mechanical marvel that allows your engine to smoothly transfer power to the wheels, enabling you to accelerate, brake, and change gears effortlessly. However, this intricate system relies on a crucial lubricant - transmission fluid - to function effectively and protect itself from wear and tear.

What does transmission fluid do?

Transmission fluid, often referred to as ATF (Automatic Transmission Fluid) or MTF (Manual Transmission Fluid), plays a crucial role in ensuring the smooth operation and longevity of your vehicle's transmission system. It acts as a lubricant, coolant, and hydraulic fluid, facilitating the movement of gears and clutches, regulating transmission temperatures, and transmitting power from the engine to the wheels. Without it, your gearbox wouldn't work.

Three primary types of transmission fluid exist:

  1. Automatic Transmission Fluid (ATF): Designed for automatic transmissions, ATF possesses special properties to withstand high temperatures, maintain viscosity, and provide frictional properties for clutch engagement.
  2. Manual Transmission Fluid (MTF) – more commonly known in the UK as manual gearbox oil: Used in manual transmissions, gear oil focuses on lubrication and wear protection, ensuring smooth gear shifting and preventing gear grinding.
  3. Continuously Variable Transmission Fluid (CVT Fluid) - for Continuously Variable Transmissions. These use special types of ATF and the same considerations apply as for automatic transmissions.

Checking and changing transmission fluid as per the manufacturer's recommendations is essential for maintaining optimal transmission performance and extending its lifespan. Some manufacturers say that their gearboxes are "filled for life" but nothing lasts indefinitely, and a transmission fluid change can often still be a very good idea.

Why transmission fluid maintenance can't be ignored

Transmission fluid plays a pivotal role in ensuring your vehicle's performance and longevity. Let's break it down:

  1. Heat Dissipation: Transmission fluid dissipates heat generated during gear shifts. Without proper cooling, excessive heat can damage internal components.
  2. Friction Reduction: It lubricates moving parts, minimising friction. Smooth gear changes rely on it.
  3. Seal Preservation: Transmission seals rely on fluid to prevent leaks. Maintaining the right levels ensures proper pressure.

Neglecting transmission fluid replacement can lead to costly repairs and dangerous situations, so ensure you are on top of your car's maintenance schedule.

What transmission fluid do I need?

Choosing the right transmission fluid for your vehicle is crucial to maintaining optimal performance and longevity. Your vehicle's owner's manual is a good starting point to find your specific car's recommended type and grade of transmission fluid, but at Opie Oils we're oil experts and can find the right oil for your car. Get in touch – we're happy to help.

Different types of transmission fluids explained

The choice of fluid depends on the type of transmission your vehicle has, there are three main types of transmission fluids:

Automatic Transmission Fluid (ATF): ATF is specifically designed for automatic transmissions and comes in many types, such as Dexron, Mercon, Type-F and many transmission-specific formulations. Getting the right fluid for your specific transmission is crucial.

Manual Transmission Fluid (MTF) / Manual Gearbox Oil: Used in manual transmissions. Available in different viscosities which determine its flow characteristics and resistance to shear. The viscosity grade recommended for a particular transmission depends on factors such as operating temperature range and load conditions. Using the correct viscosity grade is crucial for proper lubrication and performance.

Continously Variable Transmission Fluid (CVT Fluid): . Continuously Variable Transmissions are a type of automatic transmission that can seamlessly change through an infinite number of effective gear ratios between maximum and minimum values. CVT Fluid is designed especially for this type of transmission.

Signs your transmission fluid needs changing

Warning signs of transmission fluid issues

The signs that indicate the need for changing gearbox oil or automatic transmission fluid (ATF) can vary slightly between manual and automatic transmissions but often overlap. Here are some common signs to look out for:

Signs indicating the need for gearbox oil or ATF change:

  1. Visible Contamination: Check the fluid on the dipstick (if your car has one) or take an oil sample from a fill plug or inspection port on the transmission housing. If it appears dark, cloudy, or has metallic particles in it, it's likely contaminated and needs changing.
  2. Burning Smell: A burnt smell coming from the transmission area could indicate overheating and degradation of the fluid. This is often accompanied by discoloured or burnt fluid when sampled.
  3. Leaking Fluid: Puddles or stains of fluid under the vehicle can indicate a leak in the transmission system, leading to low fluid levels and potential damage if left unaddressed.
  4. Slipping Gears: If the transmission slips out of gear, struggles to engage, or shifts erratically, it could be due to inadequate fluid levels or degraded fluid affecting proper gear engagement.
  5. Strange Noises: Unusual noises such as whining, grinding, or clunking during gear changes can be a sign of low or contaminated fluid, or mechanical issues within the transmission.
  6. Delayed Engagement: Delayed response when shifting gears, especially in automatic transmissions, can indicate fluid-related issues affecting hydraulic pressure and gear engagement.
  7. Transmission Overheating: A warning light indicating transmission overheating or the temperature gauge showing high transmission temperatures can signal fluid-related issues or other transmission problems.

While many of these signs apply to both manual and automatic transmissions, some symptoms may be more pronounced in one type over the other. For example, slipping gears and delayed engagement are more common in automatic transmissions, while manual transmissions may exhibit grinding or difficulty shifting when the fluid is compromised.

The consequences of neglecting transmission fluid change

Ignoring transmission fluid maintenance can lead to:

To keep your car running smoothly, check your transmission fluid and change when necessary!

Step-by-step guide to changing transmission fluid

This guide provides general steps for changing transmission fluid, but always refer to your vehicle's specific manual for exact requirements and procedures. If you are uncomfortable with any step or unsure about the specifications of your vehicle, consult a professional mechanic.

  1. Gather your materials

Before you begin, ensure you have all the necessary tools and materials. You will typically need:

  1. Bring the vehicle to normal operating temperature

Transmission fluid drains easier when it's warm. You can warm the transmission by idling the vehicle for a few minutes or by driving around the block.

  1. Jack the vehicle up and secure it

Elevate the front of the vehicle using a jack and secure it on axle stands or wheel ramps - ensure you use the correct jacking points. Chock the rear wheels to ensure the car doesn't move while you are working.

  1. Locate the transmission fluid pan or drain plug

Slide under the vehicle and locate the transmission fluid pan on automatic transmissions or the drain plug on manual transmissions. The fluid pan is typically easy to find near the engine bay and secured with bolts.

  1. Drain the old transmission fluid

Position your fluid drain pan under the pan or plug, then use your torque wrench or ratchet to unscrew and remove it. Allow the fluid to drain completely. If your vehicle has a transmission pan, consider unscrewing one side more than the other to direct the flow.

  1. Remove the transmission pan and clean it

Once drained, remove the transmission pan completely. Check it for debris or metal shavings, which could indicate transmission damage. Clean the pan or plug it with a cleaning solvent before replacing it.

  1. Inspect and replace the transmission fluid filter

Inspect the transmission fluid filter for any damage and replace it if necessary. This is often recommended with every fluid change to ensure no contaminated material gets into the new fluid.

  1. Replace the transmission pan gasket and pan or the drain plug

Once the fluid is fully drained and the filter is dealt with, replace the pan or plug. Ensure you also replace the gaskets or O-rings to prevent leaks. Apply a layer of grease to each gasket for an extra sealant.

  1. Refill the transmission fluid

For automatic transmissions, refill with the appropriate type and amount of new transmission fluid through the filler reservoir under the hood using a funnel. For manual transmissions, you may need to use an oil pump to refill the system through a filler hole on the top of the transmission housing.

  1. Turn the vehicle on and let it idle

After adding the new fluid, turn your vehicle on and let it idle. This allows the new fluid to circulate through the transmission and coat all the internal components.

  1. Check fluid levels and for leaks

After the vehicle has idled, check the transmission fluid level using the dipstick in automatic vehicles. In manual cars, you might need to check the fluid by removing the filler plug. Make sure there are no leaks under the vehicle. If there are, you may need to tighten connections or address seals and gaskets you've replaced.

  1. Test the transmission

Before finishing, test the transmission by driving and shifting through several gears. Ensure the vehicle shifts smoothly and correctly.

  1. Dispose of old fluid responsibly

Finally, take the old transmission fluid to a recycling centre or automotive shop that accepts used fluids. Never dispose of it down drains or in a standard bin.

When to seek professional help for transmission maintenance

If you notice fluid leaks or discoloured fluid, or encounter serious issues with your transmission, it's crucial to seek professional assistance as quickly as possible. Leaks can lead to severe transmission damage, and addressing these problems early reduces the chance of further issues occurring.

What happens if a car runs out of transmission fluid?

If your car runs out of transmission fluid completely, you will find that it won't move, go into gear, or shift at all. This will be especially true for automatic vehicles. If you allow your vehicle to run out of transmission fluid, this will lead to extensive and costly repairs. If in doubt, check your transmission or take your vehicle to a professional who can check it for you.

What is a transmission flush?

When it comes to caring for your automatic transmission, you will sometimes hear the term 'transmission flush'. A transmission flush helps to ensure that there is virtually no old fluid in your transmission, torque converter, or the vehicle's cooler. It's an effective way to get rid of gunk and grime.

This process requires the use of a specialist machine that removes all the old fluid and replaces it with new fluid, sometimes including a cleaning solution to enhance the process. Carrying out a transmission flush can help prolong the life of your transmission and prevent certain issues from occurring.

As for manual gearboxes, while they don't typically require a transmission flush in the same way automatic transmissions do, they do benefit from regular fluid changes. However, the process for manual transmissions is different. Instead of a flush, manual transmissions usually undergo a fluid drain and refill procedure. This involves draining the old fluid from the transmission and refilling it with fresh fluid.

Manual transmissions have less complex hydraulic systems compared to automatic transmissions, so they don't accumulate contaminants or break down fluid as quickly. However, regular fluid changes are still important for maintaining proper lubrication and cooling within the transmission.

Extending the life of your transmission

Best practices for transmission fluid maintenance

Your car's transmission is a vital component, and proper maintenance can significantly impact its longevity and performance. Follow these best practices to keep your transmission in top shape:

      • Inspect the fluid level – for automatic transmissions there is nearly always a dipstick (with the engine running and in Park). For manual transmissions, it's usually a case of locating the oil fill plug, removing it and taking a small sample.
      • Ensure the fluid is clean, clear, and at the recommended level.
      • Use the correct type of fluid specified in your owner's manual.
      • Change the gear oil / transmission fluid and according to your vehicle's service schedule.
      • Heavy loads or stop-and-go traffic on hot days can strain the transmission.
      • Maintain proper cooling systems to prevent overheating.

Remember, preventive maintenance extends your transmission's life and saves you from costly emergency repairs. Regular care ensures smoother shifts, better fuel efficiency, and vehicle reliability.

Gearbox Oil Technical Information


For gear oil for most manual transmissions, the American Petroleum Institute set the recommendations for oil manufacturers to follow, and this applies in whatever country you're motoring. Incidentally, this is where we get the "API" rating from.

These ratings generally control the viscosity parameters and additive content for all oils produced for the automotive market.

The two most common categories are:

The Chemistry

Base stocks are important; however this is not something that all lubricant producers readily publish or would like you to know. There are three main classes:

Base stocks often determine the quality of the finished product and inevitably the price, as with everything you get what you pay for and there are many benefits of group 5 synthetic oils:


Automotive lubricants are blended with an additive package. The additives determine the characteristics of the lubricants operation and can be altered for different applications.

A common anti-wear additive used is Zinc dialkyldithiophosphate; a bit of a mouthful, so it's commonly referred to ZDDP. This type of additive literally reacts with the metal surface when the reaction energy (temperature) is high enough. This reaction layer provides a "sacrificial" surface layer that offers increased levels of protection.

Extreme Pressure Additives (EP) are used as the loading and metallic contact increase, the strength of the additive and reaction process increases. This leads to the use of a sulphur-phosphorus based extreme pressure chemical.

The EP additives form organo-metallic salts on the loaded surfaces that serve as sacrificial films to protect against aggressive surface damage. EP additives are used in oils to reduce the adhesive wear of gears and absorb the shock of the moving components making contact. These additives create a barrier or film between the moving parts, its chemistry means the oil clings on to moving parts.

Automatic Transmission Fluid Technical Information

Automatic transmission fluid (ATF) is a complex mixture of base oils and various additives designed to meet the specific performance requirements of automatic transmissions.

Base Oils

ATFs typically contains a blend of base oils derived from either mineral oil or synthetic oil sources. Synthetic base oils offer better performance in terms of temperature stability, oxidation resistance, and shear stability compared to mineral oils. The points above in the "gearbox oil technical information" hold true for ATFs too.


ATFs are blended with an additive package to enhance their performance and provide protection to transmission components. These additives include:

As you can see, there's a lot more to transmission fluid than meets the eye, and not everything in oil is oil!


ATF formulations must meet specific performance specifications set by organizations such as the American Petroleum Institute (API) and the Society of Automotive Engineers (SAE).

Additionally, gearbox manufacturers (such as Asin Warner, ZF, Honda and Getrag) specify their own standards which fluids must meet for correct operation in the gearboxes that they supply. These specifications define parameters such as viscosity, frictional properties, and compatibility with transmission materials.

Environmental Considerations

ATF formulations may also need to consider environmental factors such as biodegradability and low toxicity to meet regulatory requirements and minimise environmental impact.

How Does Automatic Transmission Fluid Differ from Manual Gearbox Oil?

Manual gearbox oil / MTF and automatic transmission fluid / ATF serve similar purposes but have distinct differences due to the different requirements of manual and automatic transmissions.

  1. Viscosity: Manual gearbox oil typically has a higher viscosity (thickness) than ATF. Manual transmissions rely on gear-to-gear contact, so MTF needs to provide sufficient lubrication and protection for the gears under high-pressure conditions. ATF, on the other hand, has a lower viscosity to facilitate smooth hydraulic operation in automatic transmissions.

  2. Friction Modifiers: MTF contains friction modifiers specifically formulated for manual transmissions to enhance gear engagement and reduce wear on the gear synchronizers. ATF may also contain friction modifiers, but they are formulated differently to optimize the performance of automatic transmission clutches and torque converters.

  3. Additives: Both MTF and ATF contain various additives to improve their performance and protect transmission components. However, the types and concentrations of additives differ between the two fluids to address the specific needs of manual and automatic transmissions. For example, oil for manual gearboxes may contain more extreme pressure additives to handle the higher loads experienced in manual transmissions.

  4. Compatibility: Manual gearbox oil is designed specifically for use in manual transmissions and may not be compatible with the materials or seals used in automatic transmissions. Similarly, ATF is formulated to meet the requirements of automatic transmissions and is unlikely to provide adequate protection or performance in a manual transmission. Using the wrong fluid is highly likely to cause a problem.

  5. Heat Dissipation: Automatic transmissions generate more heat due to the hydraulic operation of their components, so ATF is formulated with better heat dissipation properties compared to MTF. Manual transmissions rely more on gear-to-gear contact, which generates less heat, so MTF may not require the same level of heat dissipation capabilities as ATF.

Where to Buy Transmission Fluid and Gear Oil

At Opie Oils, we have a huge range of products available from the likes of Millers, Fuchs, Castrol, Motul and Red Line. We reckon we have the biggest choice in the UK (and probably further afield) so we're sure we'll have something suitable. A good place to start is our transmission fluid and gear oil product page.

We know that it's not always easy to know what gearbox oil to buy... should you need any advice on the correct fluid for your transmission - or if you've got something a bit special - then please give our experts a call on 01209 202 944 or drop us an email to and we'll be happy to help. Online chat is available during working hours too.