Because friction modification is part of the product - the transmission's shift behaviour is calibrated around the fluid's frictional characteristics.

An automatic changes ratio by applying wet clutches and bands, and how they take up depends on how the fluid behaves at the friction interface.

The additive package is engineered to give a specific friction curve as the clutch engages.

The transmission control unit's shift pressures and timings assume that curve.

A fluid with different characteristics produces shudder, flare, harsh engagement, or clutches that slip and burn.

That is why manufacturers publish named specifications and why a fluid claiming to suit many transmissions should be treated with caution unless it explicitly carries the required approval.

No. They are different products with opposite requirements in some respects, and substituting conventional ATF damages them.

A CVT transmits torque through a belt or chain gripping steel pulleys, and its fluid must provide high friction at that contact to prevent slip.

Conventional ATF is designed to let clutches slip smoothly, which is the opposite requirement.

Putting ATF in a CVT causes belt slip, scoring of the pulleys and rapid, usually terminal damage.

A dual-clutch transmission may use a dedicated wet-clutch fluid, a separate gearbox oil, or a single fluid for both depending on design - and dry-clutch DCTs are different again.

Identify the transmission type before ordering anything, because the same vehicle model is frequently sold with more than one.

Heat above all - towing, heavy loads, stop-start work and hilly routes all age the fluid far faster than the standard schedule assumes.

The fluid oxidises with temperature, and the rate rises steeply as temperature rises.

Torque converter slip generates heat, so stop-start driving and pulling away under load are the hardest conditions.

Towing adds sustained load and is the single most common reason a transmission runs hot.

Degraded fluid loses its friction characteristics and its ability to protect, so clutches slip, which generates more heat - a cycle that accelerates.

An auxiliary transmission cooler is a sensible fit on any vehicle that tows regularly, and it is far cheaper than a transmission.

It describes the service design rather than the fluid's chemistry - the fluid still degrades, and hard-worked vehicles usually benefit from a change.

Many modern transmissions have no dipstick and no specified fluid change interval.

That reflects a design intention that the fluid last the vehicle's expected life under typical use.

Fleet use is frequently not typical: higher mileage, heavier loads, towing, and longer service life than the design assumed.

Under those conditions the fluid degrades well before the vehicle is finished, and a change frequently extends transmission life considerably.

It must be done correctly - the right fluid, the right quantity and the manufacturer's level-setting procedure, which usually requires a specific fluid temperature.

By the manufacturer's procedure, which typically involves a level plug with the fluid at a specified temperature and the engine running.

Transmissions without a dipstick have a fill and level plug, usually in the sump.

The fluid expands significantly with temperature, so the level is only meaningful within a narrow specified band - often monitored with a scan tool.

The engine is usually running and the transmission in a specified selector position while the level is set.

Fluid is added until it just runs from the level plug at the correct temperature, then the plug is fitted with a new seal.

Getting it wrong in either direction causes problems: overfilling aerates the fluid, underfilling starves the pump, and both present as shift faults.

A drain and refill replaces part of the fluid and is the conservative choice; a machine exchange replaces nearly all of it and is more effective on a well-maintained transmission.

A conventional drain empties only the sump, leaving a substantial proportion in the torque converter and cooler.

Repeating a drain and refill after a short interval progressively dilutes the old fluid, which is a safe approach.

A machine exchange circulates new fluid through the transmission and displaces almost all of the old.

There is a longstanding concern about flushing a badly neglected high-mileage transmission, on the basis that it may be running on the friction characteristics of degraded fluid - opinions differ, but caution is reasonable.

Replace the filter or strainer and the pan gasket if the transmission has them; a fluid change that leaves a blocked filter achieves much less.

Colour and smell indicate oxidation and overheating; particles and friction material indicate clutch wear.

Fresh ATF is usually dyed red and is clear and bright.

Darkening to brown indicates oxidation from heat, and a burnt smell means the fluid has been seriously overheated.

A milky appearance indicates coolant, usually from a failed transmission cooler inside the radiator - a serious fault that destroys transmissions.

Fine dark particles are normal wear; visible friction material or metal fragments in the pan indicate clutch or hard-part failure.

Inspect the pan and the magnet at every service, and record what is found - the trend over successive services is far more informative than a single look.