ATF, CVT and DSG: what goes into modern gearboxes?

So, you have a used car with an automatic, imported from Europe or the US, and it is time to think about the fluid in the gearbox. The shop shelf is full of cans labelled “ATF”, “CVT”, “DCT” and “universal for all automatic transmissions”, and the salesman assures you that “ATF is ATF”. Sound familiar?

In reality, there is no such thing as a universal fluid for automatic transmissions. How to look after an automatic has already been covered in the article how to extend the life of an automatic transmission, and the main rule from there (no substitutes) matters even more for modern gearboxes.

Put simply, ATF (automatic transmission fluid) is not a standard but merely the name of a whole class of fluids, as general as “engine oil”. “ATF” on a can is rather like “medicine” on a box at the chemist’s: true, but it does not tell you what it is supposed to cure. What should be inside is decided by the approval of the gearbox or car manufacturer.

The classic automatic: from DEXRON-III to ULV

Things used to be relatively simple. Many automatics were filled with fluids to the specifications of two American carmakers (DEXRON at GM and MERCON at Ford), and these names were printed on cans as a common denominator. Hence the salesmen’s habit of calling any automatic transmission fluid “Dexron” (by the way, the colour of the fluid, as with antifreeze, says nothing about what is in it).

But these specifications did not stand still either. GM long ago replaced DEXRON-III with DEXRON-VI, a lower-viscosity fluid with a longer service life, and all DEXRON-III licences expired as far back as the end of 2006. At Ford, the counterpart from the same generation is MERCON LV. So “DEXRON-III” on a modern can usually means only “meets the requirements”, that is, the fluid maker has awarded itself the specification (more about such claims in the article on oil approvals).

And it goes further. Modern automatics have far more gears than they used to (up to ten), and to save fuel they are filled with fluids of even lower viscosity. For example, the ten-speed automatic that GM and Ford developed together got DEXRON ULV and MERCON ULV (ULV stands for ultra low viscosity). Such fluids are not meant for older automatics, and the older fluids, in turn, are no good for the new gearboxes.

Sometimes a newer fluid officially replaces an older one (GM, for instance, mostly allows DEXRON-VI in its older automatics too), but that is the gearbox manufacturer’s decision, not the salesman’s. An automatic gets only the fluid whose approval is given in the service book or the owner’s manual: not a “similar” one, not a “newer” one and not a “universal” one. The simplest option, as before, is the car manufacturer’s original fluid; failing that, only a fluid that carries its approval.

CVT: a fluid of its own for every make

The CVT is a different story altogether. A classic CVT has no gears to shift: torque is carried by a metal belt or chain running between two pulleys of variable diameter, and it is carried precisely by friction. So the fluid in a CVT must not only lubricate but also give the belt the right grip on the pulleys.

And, importantly, CVT fluids are interchangeable neither with ATF nor with each other: as a rule, each manufacturer develops its own, for its own design. What is more, they differ even within one make. For example, newer Nissan CVTs call for NS-3 instead of NS-2, and newer Toyota ones for the lower-viscosity CVT Fluid FE instead of CVT Fluid TC, and one is not used in place of the other either. Only your car’s documentation will tell you which one it needs.

Personally, cans saying “for all CVTs” make me wary: if the carmakers’ own fluids are not interchangeable, how is one can supposed to replace them all?

Dual-clutch gearboxes (DSG, DCT)

A dual-clutch gearbox (Volkswagen calls it DSG; the generic name is DCT) is essentially two manual gearboxes in one casing, run by electronics: while one gear is engaged, the next one is already lined up. And here you need to know exactly what kind of clutch your gearbox has.

1. The “dry” gearbox. The clutches work without fluid, as in an ordinary manual; such gearboxes are fitted mostly to cars with low or moderate torque. Their gear section takes manual gearbox oil, the one the manufacturer specified (and in some of them the control hydraulics have a separate fluid as well).

2. The “wet” gearbox. The clutches run in fluid; as a rule, these are gearboxes for more powerful engines. The fluid for them is a special one: it must protect the gears like a gear oil and at the same time give the clutches the right friction like an ATF. So neither ordinary ATF nor ordinary gear oil will do here.

The single-clutch “robotised” gearbox, on the other hand, which you often come across on older used cars, is not a DCT at all but, in effect, an ordinary manual with an automated clutch: the gearbox takes the gear oil the manufacturer specified, and the actuator hydraulics, if there are any, have a fluid of their own.

From the outside you may not be able to tell them apart, so do not take the word of the salesman or of an “experienced” friend: look at the documentation for your particular gearbox, by its code (it is on the plate or stamped on the gearbox casing, and a dealer can find it from the VIN).

Manual: GL-4 or GL-5

You would think everything is clear here: the bigger the number in the API class, the better. Not quite. Gear oil cans most often carry one of two API categories, GL-4 and GL-5. GL-5 is designed for the heavily loaded hypoid gears of drive axles. GL-4 these days is specified mostly for certain manual gearboxes whose synchronisers need particular frictional properties from the oil.

In other words, GL-4 and GL-5 are not a “worse” and a “better” oil but oils for different components, and GL-5 does not automatically replace GL-4: an oil that is excellent in a differential may not suit a synchronised gearbox. And a double “GL-4/GL-5” marking on one can only means that the oil maker claims both categories; whether it suits your gearbox is, once again, decided by the carmaker’s approval.

Do not smile — API itself, in its current list of categories (API 1560), has moved GL-4 to those “not in current use”, because the equipment to test it no longer exists. Yet carmakers still specify GL-4 for their gearboxes, and oil makers still print it on their cans. It is just that there is no live test behind that marking any more, and so it is all the more important to look on the label not only for “GL-4” but for your car manufacturer’s approval.

Viscosity has changed too: the SAE J306 standard for gear oils, in its 2019 edition, gained new, thinner grades SAE 65, 70 and 75 (without the letter W, so do not confuse them with the long-familiar 75W), so that the oil wastes less power. The rule here is the same as for the engine: the viscosity is the one in the documentation and no other.

“Filled for life”: true or not

Plenty of automatics, CVTs and manual gearboxes are declared maintenance-free by their manufacturers: the fluid, supposedly, is filled for life. You have to understand that this “life” is counted by the manufacturer itself: as a rule, for “European” operating conditions and, most likely, with an eye on the warranty rather than on the second owner. Meanwhile the fluid in a gearbox heats up and loses its properties with time (and mileage), whatever the glossy brochure says.

What does this mean for the ordinary car owner? If the car is new and under warranty, do what the service book says. But if you have bought a used car with an unknown history, “for life” means only one thing: nobody knows how long this fluid has already served. This is the time to go to a service station that knows this particular gearbox: have the fluid level and condition checked and, if necessary, have the fluid changed following the manufacturer’s procedure, with the fluid the manufacturer specified. For a manual gearbox the rule has not changed: check the level and top up only with the oil the manufacturer specified (more on changing gear oil).

Let me say it once more: gearbox fluid is chosen in the same way as engine oil — by the car manufacturer’s approval, not by “ATF”, “CVT” or “DSG” on the can. Repairing a modern automatic, CVT or “wet” gearbox costs incomparably more than the most expensive original fluid, so whether it is worth saving money here of all places is for you to decide.

Further reading:

• What are “original” service fluids?
• Is it worth using analogues and substitutes?
• What oils and fluids does an electric car need?