Automatic transmission fluids

A hydraulic fluid with calibrated friction, not a gear oil

An ATF does four things at once: it carries the pressure that commands the clutches, fills the torque converter, cools the clutch packs, and lubricates the bearings and the pump. What separates it from a manual gearbox oil is not viscosity but friction: in a wet clutch, friction is not to be minimised but hit exactly, and its curve has to stay stable for tens of thousands of kilometres. Which is why the specifications belong to the transmission builder rather than to a standards body — a ZF, an Aisin and a GM transmission ask for different curves, and a "universal ATF" in practice covers only older transmissions.

Fluid families and the specifications that govern them

Conventional torque-converter automatics are chosen on Dexron VI, Mercon LV, ZF TE-ML 11/14, MB 236.x, Aisin or JWS. CVT variators need dedicated fluids with a high friction coefficient between belt and pulley — the exact opposite of what a wet clutch asks for — and wet dual-clutch gearboxes need DCT fluids, which have no equivalent in the conventional ATF range; neither substitutes for an ATF, however synthetic. Hydraulic power steering usually runs on an ATF-based fluid too, and on off-road machines the category also covers the powershift and wet-brake fluids specified to Allison C4, Caterpillar TO-4 or ZF TE-ML 03, which are drivetrain rather than gearing.

How to choose, and what does not mix

Here the specification is not a recommendation but a condition of operation:

Start from the transmission's number, not the vehicle's badge: the same car can take two different fluids depending on which gearbox it was built with.
A "universal" ATF in a modern transmission with wet clutches shudders on engagement because its friction curve is wrong — not because it is a poor oil.
CVT fluid and DCT fluid substitute neither for each other nor for ATF: they are three different systems, not three variants of one product.
Do not top up with whatever is to hand. If you have not got the same fluid, use one carrying exactly the same transmission-builder approval.
On machines, check whether the fluid also serves wet brakes — there a wrong friction curve is heard at once, as a squeal under braking.