Can You Mix Dexron 6 with Dexron 3?

Adding or replacing the transmission fluid is a must-do for people who own an automatic vehicle. But you don’t add any transmission fluid you find out there. This is because when it comes to transmission fluids, it’s not always a one size fits all kind of situation. A small mistake, such as using the wrong transmission fluid, can wreak havoc on your car. Therefore, you must choose the right fluid to maintain the health and efficient operation of your car’s auto transmission.

So, can you mix Dexron III and Dexron VI transmission fluids? While you should never mix Dexron III and Dexron VI transmission fluids, it’s possible to top up Dexron VI in any proportion during repair or fluid change in past vehicle models equipped with automatic transmission. Dexron VI is compatible with previous versions of Dexron.

You can top off with Dexron VI, but a replacement after a complete drain is ideal. If you can avoid it, don’t mix up the different types of transmission fluids because of their varying composition and overall quality.

The transmission is one of the most sensitive and expensive components in your vehicle; replacing it can cost an arm and leg. Don’t put yourself in a situation where you’ll wish the automatic transmission goes back to how it was before you messed up.

Why is it Essential to Choose the Correct Transmission Fluid?

Different transmission fluids are made to only work for particular types of automatic transmissions. Therefore using the wrong kind of fluid may not work as expected and can adversely affect the performance of your transmission.

The composition of the different transmission fluids can vary wildly. Since the introduction of the automatic transmission fluid (ATF) first service-fill specification in 1949, periodic specification upgrades have been necessary. This has been part of the strategy to continually improve the automatic transmission fluid over time based on modern transmission developments.

Therefore, the wrong transmission fluid may contain friction modifiers, detergents, and other additives that may not provide the proper lubrication for the various transmission components. For example, the viscosity of ATFs such as Dexron-VI is typically lower than other earlier ATFs, but it’s ideal for modern transmission hardware.

Opting for an ATF with a greater viscosity than Dexron-VI will change the lubricity and hydraulic properties of the transmission. So based on the transmission developments, the detergents and additives in old transmission fluids may not be appropriate and can eat away the seals, gears, and bearings or even cause leaks.

This leads to costly repairs and puts everyone in a dangerous driving situation – you don’t want to drive with a faulty or malfunctioning transmission.

While some transmission fluid types will withstand higher temperatures and heavy loads, others are ideal for colder climates. Mixing them up messes with their chemical composition, resulting in transmission slipping and poor shifting, among other transmission issues. But you can save a lot of money and avoid headaches by knowing what your car needs and sticking to it.

Why It’s Not Recommended to Mix Dexron III and Dexron VI

Dexron 3

DEXRON-VI fluids and other previous ATF upgrades are backward compatible. That means you can use the latest automatic transmission fluids in earlier automatic transmission hardware.

But before you use the latest version of ATF in an old automatic transmission vehicle, it’s recommended you drain and replace the previous fluid. What happens when you mix the old with the new ATF?

Usually, the mixture between two oils and emulsion can lead to a secondary chemical reaction, which is not something you want happening inside the transmission. A combination of two chemicals inside a moving component and with heat is similar to an experiment inside the engine or transmission. It does not end well for the transmission hardware.

Understanding the Differences Between Dexron III and Dexron VI Transmission Fluids

Dexron 6

While Dexron III and Dexron VI are both from General Motors, there are significant differences between them. Understanding these differences helps you make informed decisions that prolong the life of your transmission.

  • Fluid Composition: Dexron III is made from a mixture of petroleum oil and chemical additives. But Dexron VI fluid is fully synthetic. It is a merger of synthetic base oils and more advanced additives.
  • Properties: Dexron III is thinner than Dexron VI and specifically only used in older GM transmissions. Dexron VI, on the other hand, is newer, more improved, and ideal for modern GM transmissions. The change in viscosity improves shear stress stability to maintain the fluid’s frictional properties. This results in smoother driving.
  • Oxidation stability: a vehicle under heavy usage indicates that its transmission is working hard and the transmission fluid is overheating. The fluid has to withstand oxidation under high temperatures or deteriorate. While Dexron III and Dexron VI have good oxidation stability results, Dexron VI takes the crown for better oxidation resistance.
  • Compatibility: Older GM transmissions use Dexron III and may be backward compatible with Dexron VI, but newer GM transmissions are only compatible with Dexron VI.
  • Performance: If you use Dexron VI transmission fluid in a transmission not designed to use, it could lead to potential transmission damage and reduced performance. The same also applies to Dexron III. Using Dexron VI in transmission only compatible with Dexron III could have the same negative impacts. Compared to Dexron III, Dexron VI is newer and more advanced. Dexron VI provides better shear stability, wear protection, and oxidation stability. The result is better shifting and smoother operation. Dexron VI has better performance in high temperatures and heavy loads conditions.
  • Availability: While Dexron III’s availability may be reducing in auto parts stores, you’ll still find it. Dexron III is a cheaper transmission fluid than Dexron VI.
  • Service Life: Considering that Dexron VI is of better quality than Dexron III, its service life is longer. The recommended use for Dexron VI is up to 100,000 miles, and 50,000 miles for Dexron III. But this may vary between different car models and makes.

A Dexron VI Compatibility Chart:

BrandDexron VI Compatibility
Phillips 66Compatible
Quaker StateCompatible
Royal PurpleCompatible
Super TechCompatible


Making the Right Choice for Your Car

Sometimes things are not black and white when it comes to understanding something about your vehicle. So what do you do when in doubt?

Typically, your vehicle comes with a user’s manual that details your vehicle’s do’s and don’ts. That means your vehicle owner’s manual mentions the correct transmission fluids to use. But if you are still unsure, consult a certified mechanic, who will advise you based on your car’s make and model.

Beyond Dexron III and Dexron VI: Synthetic vs. Conventional Transmission Fluids

Transmission fluid categories are two-pronged: synthetic and conventional. So, what’s the dissimilarity between synthetic and conventional fluids, and which is the best for you?

Synthetic fluids are derived from chemicals, while conventional fluids are from fossil fuels – crude oil. While synthetic fuels will cost you more than conventional fluids, they have better performance and durability.

Using synthetic transmission fluid improves shifting and better resistance to high temperatures. You also get better wear and tear protection, so your transmission lasts longer.


Is Dexron VI compatible with Dexron?

Officially, DEXRON®-VI is backward compatible. General Motors says it’s possible to replace all the previous DEXRON fluids generations with the newer ones in service. For example, you can use DEXRON®-VI approved fluids in place of DEXRON III and MERCON/ MERCON V.

What is equivalent to Dexron III?

You can use any of the several updated specifications for automatic transmission fluid by General Motors as the Dexron III equivalent. That means you can use the backward compatible Dexron VI or Ford’s Mercon V with a similar specification to Dexron. Others include ATF+4, developed by Chrysler, which also has backward compatibility, and Multi-Vehicle ATF, boasting several different ATF specifications.

Final Thoughts

Your choice of transmission fluid can be the difference between a poorly performing transmission that doesn’t last long and an efficient operating transmission that lasts through the years. Choose the correct transmission fluid to ensure the health of your transmission and smooth riding.

Mixing Dexron III and Dexron VI transmission fluids are not recommended, as it is similar to using the wrong transmission fluid. Doing so will ruin your transmission and affect its performance.

Be wary of the transmission fluid type you use. While the synthetic transmission fluid gives better performance and durability, the recommendation by your auto manufacturer carries significant weight.

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Can The Ford PowerShift Transmission Be Fixed? NO! Here’s why

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Mahir Ahmed


I'm the guy responsible for ensuring honest, informative, accurate and helpful guide to the reader.

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