Best Transmission Fluids: Keep Your Car Shifting Smoothly

Our top picks for keeping your car shifting smoothly

byKatherine Rother|
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BYKatherine Rother/ LAST UPDATED ON May 20, 2021

Changing your transmission fluid regularly is an important part of keeping your car running smoothly. Taking your vehicle to a professional for this service is usually a lot more expensive than doing it yourself. In this guide, we help you select the best transmission fluid so you can do the job right.

Best Overall
Castrol Transmax Dex/Merc ATF

Castrol Transmax Dex/Merc ATF

Enhanced friction durability makes this ATF our top pick if you’re looking for something that’s (almost) universal and works well in both power steering and transmission environments.
Compatible with lots of different vehicles requiring Dexron and Mercon; large bottle size; and great price.
The labeling is confusing, stating that it’s only compatible with domestic cars. This isn’t the case. It will work with both imports and domestics if your car takes this particular kind of Dex/Merc.
Best Value

ACDelco Dexron VI Synthetic ATF

Synthetic automatic transmission fluid that’s backward-compatible going back to Dexron III. Works in a wide variety of automatic transmissions (but check your owner’s manual).
Engineered for increased viscosity, friction resistance, and foam control, leading to easier shifting and longer transmission life. Extremely affordable.
No labeling on the bottle for what exactly it’s compatible with. Sells in too-small 1-quart bottles, which you might have to buy up to six.
Honorable Mention

Red Line GL-4 Manual Transmission Lubricant

Our top pick for your compatible manual transmission car, this Red Line transmission lubricant delivers results. If you’re looking for something that has a solid performance history, this is the pick for you.
Less slippery, low sulfur formulation; optimal heat stability; great in high-performance environments; works well in older cars; and has been known to fix third gear issues.
The formulation can act up in very cold temperatures, which translates to more difficult shifting for you until the gearbox has warmed up.

Benefits of Changing Your Transmission Fluid

  • Better fuel economy. Fresh transmission fluid has better resistance to oxidation and meets the lubricating and viscosity standards more easily. These factors can translate into better fuel economy for your car.
  • Smoother shifting. When you regularly check and service your car’s transmission fluid, it will result in the smoother movement for your gears. In the long run, it will also protect your gearbox, likely saving you an expensive repair.
  • Reduced costs. Changing your own transmission fluid can save you time and money. You can reduce the costs even further by using a synthetic transmission fluid, which has a longer overall lifespan and a couple of other features that make it last longer.
  • Extended transmission life. A well-maintained car is one that gets regular transmission fluid top-offs and a complete flush and replenishment when needed. This routine maintenance will help keep your transmission in great working order, and likely extend the life of it in the long run.
  • Rust reduction. Transmissions can be pretty finicky when it comes to stuff like rust and corrosion. Moisture, chemicals, and a whole host of other factors can wreak havoc on your transmission, so be sure to change your transmission fluid when recommended to reduce these potential problems.

Types of Transmission Fluid

Type F

Originally made for Fords that had bronze clutches, Type F fluid is one that really doesn’t get much use anymore. Bronze clutches haven’t been used since the 1970s, so unless you’re looking for transmission fluid to replenish a classic car or an antique, your vehicle likely has another type.

Dexron III/Mercon

One of the most common transmission fluids out there, Dexron/Mercon formulations work with most GMs, Fords, and imports. Check your owner’s manual for the official recommendation before assuming this is the fluid you need, though.

HFM-Style Fluids

Similar to the Dexron/Mercon transmission fluid, HFM fluids provide slightly different friction characteristics that are better suited to some types of transmissions. This fluid is on the market under a couple of different names and is used by a number of imports.

Top Brands


Founded in 1899 in Great Britain, Castrol is a global company priding itself in creating high quality automotive and industrial lubricants. One of the best brands out there, Castrol products are top notch. Some of the best sellers include the company’s Transmax DEX/MERC and the Transmax ATF CVT.


Based in Houston, Texas, Pennzoil has been around since 1913, creating innovative formulations for your automotive needs. As an international oil company, Pennzoil is one of the biggest producers of motor oil, gear oil, and transmission fluid for cars and motorcycles. Popular products include the 550042065 Dex/Merc ATF and the 5523 Type-F Transmission Fluid.


With world headquarters in Lexington, Kentucky, Valvoline has a big reach. Producing high-quality automotive lubricants and oils since 1866, Valvoline is a company you can trust. Bestsellers include the DEX/MERC formulation and the DEXRON Full Synthetic transmission fluid.

Transmission Fluid Pricing

  • Under $10: You’ll find a wide range of options at this price point. Transmission fluid doesn’t tend to be an expensive product, so you can find quality selections even at this low price point. Keep in mind that some formulations are still better than others.
  • $10 - $20: In the mid-range, you’ll find a couple of different types and options, all of which should be good quality. Sometimes it’s worth investing a little extra, especially if you have a newer or more expensive car.
  • Above $20: Not too many formulations of transmission fluid are priced here. Usually, you’ll get a bigger container for the higher price, but the quality is about on par with the mid-range, barring a few rare exceptions.

Key Features

Synthetic vs. Natural

Most transmission fluid types are available in both synthetic and natural forms, with synthetics having the slight upper hand on the market. With their improved resistance to the heat and cold, as well as sheer and oxidation, many manufacturers have already discontinued organic-based compounds in favor of synthetics. Synthetic automatic transmission fluids also tend to last longer, making them the overall recommended choice.

Fluid Type

Start by checking your vehicle’s owner’s manual. This will show you exactly which type of transmission fluid you need, so you’re prepared to pick the best one in that class. Keep in mind that most cars use either Dexron or Mercon, but some imports will require brand-specific transmission fluid.

Transmission Type

If you have a manual transmission, you’ll need a different transmission fluid than an automatic vehicle would need. Automatic Transmission Fluid (ATF) is more common on the market since most cars are automatic. That being said, Manual Transmission Fluid can be a little bit tricky to find, as it’s closer in composition to motor oil than transmission fluid. In either case, your owner’s manual will help you choose the right fluid type.

Other Considerations

  • Additives: Some synthetic- and mineral-oil-based transmission fluids also contain additives. While a number of these can be helpful for improving performance and the overall life of both the transmission and the fluid itself, others can be harmful. Monolec is one example of a beneficial additive you can look for.
  • Age of Car: A number of older cars will run better on mineral-oil based transmission fluid, and introducing a synthetic ATF could actually lead to damage. If you have an older vehicle, check which fluid it’s been running on, and don’t switch fluid types to ensure the best performance.
  • Bottle Size: Depending on your vehicle, it could require 6 or more quarts of transmission fluid for a complete flush and change. Keep that in mind when comparing brands, and opt for multi-packs to save money.
  • Quality: Stick with big name brands and respected smaller ones to ensure you’re getting a high-quality product. Knock-offs and cheaper import products might be tempting, but they could end up costing you more money in the long run, for example, if a transmission repair becomes a necessity.

Best Transmission Fluid Reviews & Recommendations 2021

This Castrol automatic transmission fluid is our top pick. It features enhanced friction durability for smooth shifting and has repeatedly been consumer tested, showing high-performance results. Depending on your exact make and model, Transmax could work for you, as it meets and satisfies the Dexron, Dexron III, Dexron-III H, IIE, and II requirements, as well as all Mercon requirements.

Another use for this ATF is in your power steering if you’ve got a Mercon-requiring vehicle. That makes it an ideal choice if you’re looking to top off your transmission and power steering fluids with one product. The large bottle size is also of benefit since you’ll likely only need one, saving you plastic waste in the process.

As always, make sure to check your vehicle’s owner’s manual to ensure compatibility. The only thing we didn’t like about this particular fluid is confusing labeling. On the label, it mentions that it’s for domestic cars, which is incorrect, since this type of fluid can work in imports as well, so long as they require Dex/Merc fluid.

Companies like ACDelco are constantly refining and re-releasing the formulas for their automatic transmission fluid. Dexron VI, as you might expect, is the sixth implementation of the Dexron formula, but ACDelco strikes a blow for usability by making it backward-compatible with every formula going back to Dexron III. If you’ve used III, IV, or V in your transmission before now, you can use VI without any detrimental impact.

Dexron VI is cheap and it works. After the flush, buyers report that their park through drive shifting becomes much smoother, a consequence of Dexron VI’s improvements to friction tolerance and viscosity. All that and it cleans out rust, too. All in all, this is a great budget choice to extend the life of your transmission.

Really, the only bad thing we’ve got to say about this transmission fluid is that we wish it was sold in bottles larger than one quart. That creates a lot of hassle for buyers, not to mention unnecessary plastic waste in the environment.

Manual transmissions work a bit differently than automatic ones, which means they also need a different kind of transmission fluid. Red Line’s GL-4 transmission lubricant is our top pick for your manual car (just make sure it meets your specs). The formulation is less slippery and low in sulfur, which makes it compatible with brass synchronizers, and it meets the requirements of 70W, 75W, 80W, SAE 30, 5W30, and 10W30 motor oil. It’s also available in a higher viscosity formulation (MT-85 and MT-90).

With optimal stability at high temperatures and in performance applications, this transmission lubricant does especially well in older muscle cars, as well as sporty Hondas, Dodges, and Acuras. It’s also been known to solve third gear issues in some cars, all while improving the overall glide of the transmission for smooth shifting. Most manual transmission cars require about two quarts of fluid, so plan accordingly.

Overall, this is a great quality product with promising performance results. The only problem we noticed in the formulation is that it appears to act up a bit in very cold temperatures. That means more difficult shifting until the gearbox has warmed up enough to thin out the lubricant.

Royal Purple Synchromax is an alternative choice for drivers with manual transmissions. Specifically, it’s best for those manual-shifting vehicles whose owner’s manuals recommend that they be filled with automatic transmission fluid. This is a common situation—manual vehicles from Hondas to Ford trucks to Jeeps come with that recommendation—and if it applies to you, Royal Purple Synchromax is a practically ideal product.

Let’s start with the positives. Royal Purple Synchromax is engineered to cut down on parasitic power loss, and almost always achieves that goal with flying colors. You might notice not just an increase in horsepower, but also smoother shifting, especially compared to OEM transmission fluid. In fact, don’t be surprised if your manual transmission starts feeling close to brand new.

As for the negatives: Synchromax contains some ingredients classified as unsafe under California Proposition 65. Be careful to only use it in a well-ventilated area, and wear a dust mask and gloves while you work. Outside of health concerns, the main drawback is that not everybody notices the same benefits from using this fluid—there’s a chance that you’ll add it to your transmission and not notice any real difference.

Ravenol is a German company, and its transfer case fluid works most effectively in German cars: It matches OEM specifications for Volkswagens and BMWs, along with Land Rovers, Nissans, and some others. This fluid is designed for electronic active transfer cases, so you’ll get the best results if your vehicle has a modern electric system.

Ravenol DTF-1 transfer case fluid is a powerful lubricant with a low friction coefficient that will cut down on both immediate power and loss and long-term wear. For a fraction of the cost of a BMW-branded fluid, this is practically an identical product, excellent at reversing transfer case slippage, and smoothing out shifts.

Be warned before you use it that it works on a smaller range of cars than some other products on this list, and also contains potentially toxic chemicals. Also, non-European users might have trouble with the cap assembly, which contains an inner cylinder you need to deploy before inserting the bottle into the transfer case.

Mobil is part of one of the world’s most recognizable oil companies, so it stands to reason they’d be a strong contender for the title of best automatic transmission fluid. This particular formula, ATF 3309, is a cheap, powerful way to quiet your transmission down and make your vehicle as drivable as when it was brand new.

To begin with, it works in a wide range of vehicles—prominently Toyota, Lexus, and Volvo models, but others as well. It’s also affordable on a budget, with 12 quarts (enough for two to four transmission flushes) available for bulk purchase. It’s close enough to factory ATF to not pose a risk, but works significantly better for smoother shifting and longer transmission life. In particular, this is one of our favorite fluids for treating a transmission that’s starting to shudder.

Anything for buyers to beware of? Well, that bulk deal is a double-edged sword: If you only need a few quarts, you’ll be paying twice as much for them. Also, this is not a synthetic transmission fluid, so if you’ve found that synthetic works better in your transmission, we don’t recommend switching back to Mobil after that.


  • Check the level of your transmission fluid regularly. If it’s low, don’t forget to top it off to the indicated mark with the recommended fluid type to keep everything running smoothly.
  • To properly check your transmission fluid levels, make sure that the engine is warm since this type of fluid expands when hot. Adding a little fluid at a time will help prevent overflowing.
  • Changing your transmission fluid DIY-style can get messy. Make sure you have a large enough catch pan to keep the mess off your garage floor.
  • About 4 to 6 quarts of new transmission fluid is needed to do a proper change. If you’re doing it yourself, make sure you know how to properly drain and clean all necessary components to avoid damage to your transmission.
  • Remember that any transmission fluid change should be accompanied by a new filter and a pan gasket or RTV sealer, depending on your make and model. This will ensure that all components work seamlessly again.


Q: Why do I need to change my car’s transmission fluid?

A: Transmission fluid, like all other fluids in your car, deteriorates in quality over time. The more you use your car, the faster that rate of deterioration will be. In order to keep your car running smoothly, and to avoid a very costly transmission box repair, you should change your transmission fluid.

Q: Can I check if my transmission fluid needs changing?

A: Yes! It’s actually a pretty easy process: Keep your car running for about five minutes, then go ahead and pull out the transmission fluid dipstick and wipe off this first sample on a clean rag. Then, reinsert the dipstick and pull it out again. This time, inspect the appearance of the transmission fluid on the dipstick if it’s extremely dark or smells burnt, then it’s time to change the fluid.

Q: How often do I need to change my car’s transmission fluid?

A: The timing of transmission fluid change is a bit of a debate. There are no clear-cut guidelines as to when the best time is. Your car’s owner’s manual will have a recommendation on when to do this, but a good rule of thumb is somewhere between 30,000 and 60,000 miles. The rougher you drive your car, the more often a change will be needed.

Q: Can I change the transmission fluid myself?

A: Yes, you definitely can DIY. It’s a little bit more complicated, but doable if you’ve got some tools and a bit of car knowledge. With a little preparation, it’s a task you can complete in an hour or two. Follow these instructions if you’re keen on doing it yourself.

Q: Are transmission fluid and motor oil the same thing?

A: With all the different types of fluids out there, it’s easy to get them all a little mixed up. Transmission fluid and motor oil are not the same things the former is used to keep your car’s transmission (aka the gears) running smoothly, while the latter makes sure the motor functions well.

Final Thoughts

Our top pick for the best transmission fluid is the Castrol Transmax Dex/Merc ATF. It works for the majority of domestic cars out there and delivers great results.

If you’re looking for something even more wallet-friendly, check out the ACDelco Dexron VI Synthetic ATF. It has a great value to performance ratio and can likewise be used in many different makes and models.

Why Trust Us

Our reviews are driven by a combination of hands-on testing, expert input, “wisdom of the crowd” assessments from actual buyers, and our own expertise. We always aim to offer genuine, accurate guides to help you find the best picks.

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