Best Transmission Fluids: Keep Your Car Shifting Smoothly (2024)

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.

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