allison transmission , diesel , duramax , transmission fluid

What’s The Best Transmission Fluid For Your Allison Transmission?

What’s The Best Transmission Fluid For Your Allison Transmission?

What’s The Best Transmission Fluid?

The Allison transmission in your Duramax truck is a carefully crafted, strong, purpose-built piece of equipment that can endure many miles of trouble-free use. The Allison transmission is made to last, but it's not a "set it and forget" affair. Unlike what you see on TV, it needs specific maintenance and parts to keep running for years.

The automatic transmission fluid (ATF) that circulates through your Allison transmission is one of the most critical parts of the Allison transmission, and it is something that is ostensibly simple as fluid, but it does contain a lot of engineering magic.

When it comes time to maintain your Duramax truck, you’re probably wondering what the best transmission fluid for your Allison transmission is. Don’t worry, we’ve got you covered!

Let’s get into the thick of it and talk all about automatic transmission fluid!

What Is Transmission Fluid?

Transmission fluid is essentially a base (synthetic or conventional) oil with specific additives that are designed to work with hardworking transmissions. Among the additives that you can find in automatic transmission fluid are:

  • Friction modifiers that help the fluid stick to metal better
  • Anti-wear additives
  • Rust and corrosion inhibitors
  • Detergents, dispersants, and surfactants (which protect and clean metal surfaces)
  • Seal swell additives and agents
  • Anti-foam additives and antioxidant compounds to inhibit oxidation and "boil-off"
  • Cold-flow improvers
  • Extreme temperature thickeners
  • Gasket conditioners
  • Petroleum based dye

ATF is way more than just oil, in fact, it is a carefully engineered piece of the automotive technology puzzle that plays an absolutely crucial role in your Allison transmission.

What Does Transmission Fluid Do?

Since the advent of the automatic transmission, utilizing a technologically specific fluid has played an integral role in the evolution of this crucial component. As transmission technology has advanced, the need for more durable, lower viscosity fluids has come to the forefront of transmission tech. Modern fluids need to be extremely durable and possess characteristics that allow for modern vehicles to achieve high levels of fuel economy. For those that drive their heavy-duty truck out on the open highway, this commitment to fuel economy is always appreciated.

There are two primary functions of transmission fluid, but they vary slightly depending on the type of transmission in your vehicle. If you run a manual transmission, the transmission fluid’s primary goal is to cool and lubricate the internal components of your transmission. If you run an automatic transmission, such as those made by Allison, your transmission fluid performs as both a lubricant and as a hydraulic fluid that is necessary for the self-shifting duties of your transmission.


Part of what makes Allison transmission so incredible is their commitment to offering a product that is put through the gauntlet of commercial usage. From semi-trucks that are barreling down the highway with 70,000 pounds of weight behind them to commercial tractors that operate out in the fields; the Allison transmission In your Duramax truck benefits from the research and development that goes into these hardcore applications. Among the biggest challenges with all mechanical equipment in these scenarios is the generation of heat and friction under load.

ATF is designed to provide optimal lubrication to the metal components of your Allison transmission, even under heavy usage conditions like heavy acceleration or towing a large load. Reducing friction between metal parts reduces wear and extends the working life of your Allison transmission. If your transmission fluid is dirty or worn, it will not cool lubricate as well and could also lead to long-term failure.

Hydraulic Fluid

Hydraulic Planetary Gearset Automatic Transmissions use automatic transmission fluid to operate a torque converter that is necessary to get a vehicle moving from a dead stop. This ingenious, if often overlooked, piece of automotive technology has ATF at the absolute center of the party. Without the proper level and viscosity of fluid, automatic transmission functionality will be severely affected, and your transmission could suffer long-term damage.

Check out this fantastic video for an explanation of how a torque converter works.

The automatic changing of gears is another crucial element of automatic transmission fluid. Depending on speed and driver input, the hydraulic system in the Allison transmission operates clutch packs, valves, and pistons to correctly select the right gear for the right scenario. Improper transmission fluid, dirty transmission fluid, or worn transmission fluid can drastically affect the proper operation of the Allison automatic.

What makes transmission fluid good or bad?

When it comes right down it, good automatic transmission fluid for your Allison transmission needs to possess a blend of the following characteristics:

  • Highly levels of friction reduction
  • High levels of heat modulation
  • High levels of durability
  • Long service intervals
  • Synthetic blend

First off, the transmission fluid in your Allison transmission needs to be able to cope with the intense conditions that your Duramax truck thrives in. Putting your transmission under load means that friction is also increased. Increased frictions, means increased heat, and increased heat means increased wear and tear on our transmission! Good automatic transmission fluid will have stable friction protection characteristics.

The ability of your transmission to dissipate heat is crucial to its long life. Allison transmissions employ a transmission cooler that is designed to cycle/cool transmission fluid and return it back into the transmission for optimal cooling. If the fluid does not have stable heat modulation capabilities, it can cause heat build-up in the transmission. This condition can lead to long-term failure.

Durability and service life go hand in hand when it comes time for transmission fluid in your Allison transmission. As your transmission fluid moves through its service life, it begins to break down into individual compounds and also gets worse with debris. This drastically reduces its effectiveness as a barrier against heat and friction. Good transmission fluid is highly durable, and resists break down, which affords owners a longer service life.

As mentioned earlier, there are conventional and synthetic blends of automatic transmission fluid and much like engine oil, synthetically produced automatic transmission fluid has been shown time and again to provide long high levels of protection and long service life.

As a baseline, the best automatic transmission fluids are synthetic blends like GM’s own advanced ACDelco Dexron VI fluid.

What’s the best transmission fluid for my Allison transmission?

So, what is the best automatic transmission fluid for your equipped Allison transmission, Duramax truck? The answer here is actually quite simple: if you want the optimal, manufactured performance, and durability then stick with the factory specified fluid!

The factory on Duramax / Allison equipped trucks is ACDelco Dexron VI fluid and it represents not only a good value but is carefully engineered to provide optimal protection and long service intervals that you expect out of high quality, synthetic automatic transmission fluid.

This specification was introduced in 2005 to replace Dexron IIIH and according to General Motors, DEXRON-VI fluid is a significant improvement in comparison to DEXRON-III (H) fluids. Fluid life is significantly longer due to improvements in oxidation stability, friction durability, shear stability, and foam resistance over current DEXRON-III fluids.

This product is backed by factory engineering and adheres to a strict level of quality control that makes its competitive price that much more appealing vs. brands like Transynd. Remember, General Motors is not just going to put any transmission fluid into their Heavy-Duty trucks because these vehicles are backed up by robust factory warranties! If the vehicles fail, GM is on the hook for expensive repairs, so why would GM put in an inferior transmission fluid?

Simply put, there really isn’t much to complain about with ACDelco Dexron VI fluid. It offers fantastic protection, long service intervals, factory backed engineering, and a reasonable price point.

It’s our pick for the best automatic transmission fluid for your Allison transmission.

The Diesel Experts

At Merchant Automotive, we know a thing or two about the unique requirements of Allison transmissions and the people that love them because we’ve made it our passion for the past 17 years. We’ve made it our business to be the experts around these unique trucks and accumulate a wealth of technical and practical knowledge that allows us to offer some of the most unique products in the Allison / Duramax marketplace.

If you drive a Duramax truck, the team at Merchant Automotive has what you need.

What is transmission fluid and how does it work? What makes a good/bad fluid? What fluid is best for Allison transmission?

ACDelco Dexron VI fluid is the ideal choice for Duramax/Allison equipped trucks. It's a great value and is designed to offer optimal protection and long service intervals.