ATF stands for automatic transmission fluid: the oil inside an automatic gearbox. Many people call it hydraulic oil because, under pressure, it carries the engine's force to the transmission and engages the gears. To choose one, look in the manual for the fluid code (such as Dexron VI) and buy a bottle recommended for that code; a red color and a "for automatic transmission" label are not enough.
What is ATF oil used for?
In a manual car, your left foot presses the clutch and your hand changes gear. In an automatic, the oil does that job, and it also lubricates and cools the transmission. It has five jobs:
It carries the engine's power to the transmission. The turning force of the engine is called torque, and the torque converter is the oil-filled coupling between the engine and the transmission. It works like two fans facing each other, one switched on, making the other spin with nothing but its wind (Figure 1). The bladed wheel that turns with the engine (the impeller) throws the oil against another bladed wheel (the turbine), which is connected to the transmission.
The turbine does not have to keep up with the impeller: with the car stopped and in gear, it stays still while the impeller turns; on the move, it turns a little slower. This slip lets a stopped car stay in gear without stalling the engine, and it also produces heat. At steady speed, many transmissions close a lock-up clutch, which ties the two bladed wheels together and ends the slip.
Figure 1. The torque converter works like two fans facing each other, with oil in place of air. The difference: the oil circulates in a closed loop, and there is a third bladed wheel, the stator, which sends the oil back to the impeller in the direction of rotation and, in doing so, multiplies the force when pulling away.
It delivers pressure to the clutches. A pump that turns with the engine pushes the oil, and the valve body (a block of passages and valves) chooses where it goes. In current transmissions, the electronic control unit gives the order; in older ones, valves moved by the oil's own pressure do.
The oil reaches the piston of the requested gear, and the piston presses the clutch pack. It is a stack of steel discs alternating with friction discs, bathed in oil, like the clutch of a manual car, only inside the transmission. Once pressed, the discs hold part of the gears, and that is how the gear changes.
It controls the friction of the discs. When the piston presses, the discs are still turning at different speeds, and they have to reach the same speed together, without a jolt. What gets this right are the friction modifiers, a type of additive (a chemical, in a small dose, that gives the oil a quality it would not have alone). They hold the friction at the right level while the discs slow down, until the end of the engagement.
Without them, the friction jumps at the end and the shift jolts; with modifiers of the wrong type or dose, the friction may be too low, and the clutch slips.
It carries the heat away. The oil, heated by the slip, goes through the cooler, the radiator for the transmission oil. Too much heat speeds up oxidation: the oil reacts with oxygen in the air and forms acids, sludge and varnish.
It lubricates. On gears and bearings, the oil keeps a thin layer, the oil film, between the parts, which keeps metal from touching metal.
Figure 2. The path of the ATF in a simplified diagram, with the converter also in diagram form, and the numbers from the list: (1) converter, where the oil leaves the impeller, passes through the turbine and returns through the stator; (2) pump and valve body; (3) clutch discs; (4) cooler; (5) lubrication, before returning to the sump.
How this shows up in the shop and in the fleet: slow, harsh or jolting shifts; vibration when engaging or at steady speed, when the converter locks; engine revs that rise without the car keeping up (the clutch slips); a transmission hotter than normal.
Is ATF oil hydraulic?
Yes, by the work it does. A hydraulic system is one in which a liquid transmits force. In a car's brake, the force goes by pressure: the foot pushes a liquid, and the liquid pushes the pad. In the transmission, the pump, the valve body and the piston do the same. And in the converter, the force passes through the movement of the oil itself, like the wind between the fans in Figure 1.
The brake is only an image. Common brake fluid (DOT 3, DOT 4 and DOT 5.1) is made of glycol, a type of alcohol that mixes with water. It does not mix with oil, it draws moisture from the air and it does not lubricate the transmission. And oil in a brake swells the rubber. One never goes in place of the other.
ATF also does not swap places with industrial hydraulic oil, for two reasons:
Friction. The anti-wear hydraulic oil used in presses and plastic injection machines (of the HLP class, the one with an anti-wear additive) protects pumps and valves, but it does not have the friction modifiers that a disc clutch needs.
Thickness. Viscosity is how much an oil resists flowing: honey flows slowly, water flows fast. The "68" in hydraulic oil 68 is the ISO VG grade (from viscosity grade), the oil's viscosity class at 40 °C, and ATF is much thinner than that.
When the question is "hydraulic oil 68 or ATF", each stays in the system it was specified for: ATF goes into an industrial hydraulic system, or a hydraulic jack, only if the equipment manual calls for it. Many hydraulic power steering systems in cars, on the other hand, use ATF when the manufacturer says so. Industrial types of hydraulic oil have their own article.
For those who need the numbers: HLP is the class in DIN 51524 part 2, which corresponds to HM in ISO 6743-4. By ISO 3448, ISO VG 68 runs from 61.2 to 74.8 cSt at 40 °C, and ISO VG 32 from 28.8 to 35.2 cSt. Modern ATF sits in the ISO VG 32 range (Dexron VI and Mercon LV at its lower edge; their low viscosity shows up more at 100 °C and in the cold), and some reach ISO VG 22 (19.8 to 24.2 cSt).
In ISO 6743-4, the HL oils (with a rust and oxidation additive), HM (with an anti-wear additive) and HV (with a high viscosity index) belong to the hydrostatic application, in which force passes by pressure. ISO 11158, which specifies these classes, does not cover ATF, which is defined by the carmakers and by a Japanese standard (below).
What are the types of ATF oil?
The types of ATF follow the transmission, and each carmaker asks for its own fluid, written in a specification. A specification is a list of tests with a name, which is the code in the manual, such as Dexron VI, Mercon LV or Toyota WS. Like every lubricant, ATF is base oil (the liquid that makes up most of it) plus additives; the article on what base oils are explains. In practice, there are four types of fluid, one per family of transmission:
ATF for cars and light utility vehicles. The one from earlier generations is thicker and, in general, mineral-based (oil refined from petroleum). The one from newer generations is thinner (LV, from low viscosity) and, in general, synthetic-based or deeply refined mineral.
Heavy-duty ATF. For automatic transmissions in trucks and buses, with a specification from the transmission maker, such as Allison C-4 (Allison is a transmission manufacturer).
CVT fluid. For the continuously variable transmission (CVT), the transmission without stepped gears, with pulleys and a steel belt.
Dual-clutch transmission fluid. For the transmission with two clutches that take turns on the gears; some of these transmissions call for a fluid of their own, not a common ATF.
There are also multi-specification fluids: a single bottle, recommended for several specifications, which simplifies stock for shops that serve many brands.
If they all do the same five jobs, why can't one stand in for another?
Why doesn't one ATF specification replace another?
Because the carmaker tunes the transmission to one fluid (the force the clutch withstands, the time of each shift, the rubber of the seals) and writes what it needs in the specification.
There is also a public ATF standard, from no carmaker: JASO M315, from the Japanese automotive standards organization, with classes 1A and 1A-LV. It may appear on some bottles, but the code in the manual is what rules.
The substitute for an ATF is another fluid recommended for the same specification, or for a specification that the carmaker itself declares a substitute; being newer is not enough. Three requirements weigh directly on the shift.
Viscosity and shift time
A fluid that is too thick, or cold, is slow to run through the passages: the piston completes its stroke and presses the discs, but the shift may be late. A fluid that is too thin escapes through the clearances (the small gaps in the pump and valves): pressure drops, the piston fills slowly and presses the clutch with less force, and it may slip. The electronic control unit learns from the shifts and corrects, but only within the range calibrated for the right fluid.
Every oil thickens in the cold and thins in the heat, and the viscosity index tells by how much: the higher the index, the less the oil changes. That is why the specification fixes the viscosity at more than one temperature, from the deep cold of a start-up to the hot transmission, and not at just one, like the ISO VG 68 of hydraulic oil, measured at 40 °C.
Figure 3. Too thick, the fluid is slow to flow, but the piston completes its stroke and presses the discs; too thin, the oil escapes through the clearances, pressure drops, the piston fills slowly and the clamping force is weak.
Controlled friction in the clutch discs
The friction modifiers of each specification are dosed for the friction material of that transmission. With the wrong chemistry, the disc slips (engine revs rise and the material heats up) or the friction jumps at the end (jolt and vibration). In both cases, the friction material wears out faster.
Figure 4. The same clamping force, three results: with the right friction, the discs reach the same speed together; with too little, they slip; with friction that jumps at the end, they jolt.
Compatibility with the seals
Seals and O-rings (the parts that seal the shafts and passages) are tested with the fluid of the specification. An incompatible fluid swells, shrinks or hardens the rubber, and the leak, which starts small, takes pressure away from the clutches.
For those who want the technical detail: Dexron VI and Mercon LV are thinner than the earlier generations at 100 °C and in the cold. The JASO M348 method measures the friction during the engagement, in the middle and at the end, when the discs almost stop, and the friction needed to make them slide again once locked (static). If the friction at the end goes well above that in the middle, the shift jolts.
JASO M349 measures how long the fluid keeps its effect against shudder. The specifications also test oxidation and foam. And the additive that improves the viscosity index, a polymer with long molecules, is cut by use (shear loss): the fluid thins over time.
What is the difference between ATF and CVT fluid?
On a bicycle, the chain jumps from sprocket to sprocket, in steps. In a CVT (continuously variable transmission), two pulleys open and close, and a steel belt or chain moves up and down on them without steps. All the force passes through the friction of the belt's steel on the pulley's steel. That is why CVT fluid needs high friction between steel and steel, higher than that of ATF.
Figure 5. In the CVT, the pulleys change diameter and the belt moves up and down without steps; on the bicycle, the chain jumps from sprocket to sprocket.
CVT fluid, then, looks after two kinds of friction. In the clutches (forward, reverse and converter lock-up, which many CVTs also have), it needs the controlled friction of ATF; between belt and pulley, it needs high friction. With ATF, the belt slips. That is why one does not go in place of the other, unless the carmaker expressly says so.
Watch out for hybrids: some transmissions in these cars also carry the CVT name, but work in another way, with gears and electric motors bathed in the fluid, with no belt or pulleys. The manual of these transmissions calls for ATF, and that is what counts.
The dual-clutch transmission and the automated manual (in which a mechanism changes gear for the driver) call for the fluid the carmaker recommends, which may not be ATF; some use two fluids, one for the gears and another for the hydraulic module that operates the clutches. In these cases, talk to the Kelpen Oil Technical Team.
For those who want the technical detail: JASO M358 is the public test method for the metal-on-metal friction of belt CVT fluid (block on ring). It measures that friction without creating a class; for that reason, no bottle is recommended for JASO M358.
How do I find the correct specification for the transmission?
The specification is in the vehicle manual, and four steps avoid the most common mistakes:
Check the manual. The code appears in the maintenance or capacities section; some transmissions also show it on the dipstick or on a label on the engine.
See whether the carmaker accepts another. Only what the maker of the transmission declares counts, in the manual or in a bulletin. GM, for example, accepts Dexron VI in its own transmissions that called for Dexron III, which does not extend to other brands. In a transmission of another brand that calls for Dexron III, use the fluid recommended for Dexron III. The reverse never holds: a fluid of an earlier generation does not go into a transmission that calls for the new one.
Check the code on the bottle. The fluid must be recommended for the code the manual calls for; "suitable for automatic transmission" is not enough.
Consider the use. Towing, mountain roads and heavy traffic heat the fluid and speed up oxidation; many manuals have a change interval of their own for this severe service.
For those who need the numbers: GM stopped authorizing new bottles with the Dexron III name at the end of 2006 and keeps Dexron VI, launched in 2005 for model-year 2006 cars; Ford ended the original Mercon in July 2007 and moved those who called for it to Mercon V. These old codes remain in manuals, and the fluid is still chosen by the code.
With the code in hand, what is left is choosing the bottle.
What is the best ATF oil for each transmission?
The best ATF oil is the one recommended for the code in the manual, with no approximation. We at Kelpen Oil formulate the Supramatic line: three fluids for conventional automatic transmissions and one for CVT. If your manual calls for Dexron III, the fluid to buy is the one recommended for Dexron III (in the table, DX III M); confirm in the manual, and the Kelpen Oil Technical Team helps you choose. If you have another specification, such as Mercon V, talk to the Technical Team, who will check the manual. The table shows which Supramatic is recommended for each code.
| If the transmission manual calls for… | Use | Base |
|---|---|---|
| Dexron III, Ford Mercon or Allison C-4 (earlier generations and heavy duty) | Supramatic DX III M | Highly refined mineral |
| Dexron VI | Supramatic DX VI | Synthetic |
| Mercon LV, Toyota WS (JWS 3324), Hyundai SP-IV, VW TL 052 162-X/Y and TL 052 540-B, ATF DW-1, MB 236.12/14/15/16/17, ATF J3 and Dexron VI; or several brands in the same shop or fleet | Supramatic MULTI | Synthetic, in a single bottle |
| CVT fluid for light vehicles, with steel belt or chain: Toyota CVTF TC and CVTF FE, Nissan NS-1 and NS-2, Honda HMMF and HCF-2, Mitsubishi CVTF-J1, Subaru ECVT and iCVT, Daihatsu AMMIX CVT, Suzuki CVTF NS-2 and CVT Green 1, Chrysler and Jeep NS-2, Mini Cooper EZL799 | Supramatic CVT | Synthetic |
| Another specification, hybrid transmission or dual-clutch transmission | Consult the Kelpen Oil Technical Team with the specification from the manual | Each specification calls for its own fluid |
The DX III M, the DX VI and the MULTI are recommended also for hydraulic power steering, according to the manual, and are compatible with the seals, O-rings and other rubber or plastic parts of the system. The MULTI also meets the public standard JASO M315, classes 1A and 1A-LV. The Supramatic CVT is not recommended for CVT units in hybrid vehicles; many hybrids call for ATF.
How does Kelpen Oil check Supramatic and used ATF?
In our own laboratory in Arujá (SP), we check the new oil: no batch ships without approval, and a retained sample of each batch (a bottle kept for future comparison) is archived.
The same laboratory also analyzes the ATF that has already run in the customer's transmission, as a paid service, to see whether it has oxidized, taken in water or been mixed with another fluid.
This analysis has a limit: it does not measure the effect left in the friction modifiers (only a friction test shows that, and the laboratory does not run one) or the wear of the transmission. That is why the interval in the manual still holds even with a good analysis. See also the lubricant laboratory tests.
| What we measure in the customer's used ATF | What it reveals | Reference method |
|---|---|---|
| Viscosity at 40 °C and 100 °C | A drop points to mixing with a thinner fluid or to the polymer being cut by use; a rise points to oxidation or mixing with a thicker fluid | ASTM D7042 |
| Acidity (total acid number, or TAN) | How much the fluid has oxidized | NBR 14448 |
| Water | Ingress of water or coolant, by condensation or a cooler failure | Coulometric Karl Fischer |
| Mixing with another fluid: infrared spectrum (the fingerprint of the molecules) and additive elements, such as zinc and phosphorus | Mixing with another product or consumed additive | ASTM E2412 and the NBR 14786 method |
Frequently asked questions
Can I mix ATFs of different specifications? Avoid it: the mixture is no longer guaranteed to suit any of them. Top up with the same fluid. To change fluids, ask for a complete change: draining the sump removes only part of the oil, and the rest, in the converter and the cooler, mixes with the new one.
Does ATF work for power steering? It does when the vehicle manufacturer calls for ATF in the steering. Supramatic DX III M, DX VI and MULTI are recommended for this use as well; some steering systems call for another fluid, and the manual comes first.
What is the difference between Supramatic DX VI and Supramatic MULTI? DX VI is recommended for transmissions whose manual calls for Dexron VI. MULTI is recommended also for the other specifications in the table, such as Mercon LV and Toyota WS, and meets JASO M315 1A and 1A-LV; with it, a shop serving many brands keeps fewer fluids in stock.
Can I use CVT fluid in an automatic transmission? No, unless the carmaker says so: CVT fluid is made to hold the steel-on-steel friction of the belt, and the clutches of a conventional transmission expect the friction of their own specification.
When should I change the automatic transmission fluid? At the interval in the manual, and at the severe-service interval if the vehicle tows, climbs mountain roads or runs in heavy traffic. Laboratory analysis helps track the fluid, but it does not replace that interval.
Glossary
ATF: automatic transmission fluid, the oil that carries the force, applies and controls the clutches, lubricates and cools the transmission.
Torque converter: oil-filled coupling between the engine and the transmission.
Valve body: block of passages and valves that sends pressurized oil to each clutch.
Clutch pack: stack of oil-bathed discs that the piston presses to connect or hold part of the transmission.
Friction modifier: additive that sets the friction of the discs so the shift is smooth, with no friction spike at the end, which makes it jolt, and without letting the disc slip.
Specification: list of tests, with a name, that the carmaker defines for the fluid of its transmission.
Viscosity index: number that shows how much viscosity changes with temperature.
CVT: continuously variable transmission, with a steel belt between pulleys, without steps.
Talk to the Technical Team
The Kelpen Oil Technical Team evaluates the specification in the manual, the type of transmission, the hydraulic power steering, the fleet's use and the volume, and confirms the right Supramatic. For the fluid already in the transmission, the laboratory in Arujá (SP) performs in-service oil analysis, as a paid service. Save this article or send it to whoever looks after the maintenance of your fleet or shop.