Automatic transmission fluids, abbreviated as ATF from the English „Automatic Transmission Fluid“, are special liquids designed for use in vehicle automatic transmissions. Unlike conventional gear oils, they do not only serve to lubricate moving parts. They also function as a hydraulic fluid, which transmits engine power via a hydrodynamic torque converter, a device enabling smooth power transfer without a rigid mechanical connection. Furthermore, ATF cools transmission components, removes impurities, and ensures proper friction for clutch plates, which shift individual gears. There are many different specifications for these oils, such as Dexron, Mercon, or ATF+4, which differ in their properties, such as viscosity, meaning the degree of internal friction of the fluid, and the composition of additives. Using the wrong type can lead to malfunction or damage to the transmission, so it is always necessary to adhere to the specification prescribed by the vehicle manufacturer.