Maintenance of your automatic gearbox is essential for long transmission life. Thermal degradation of your gearbox oil is a contributing factor in delayed shifting, harshness and eventually failure of gearbox components. A modern automatic gearbox can cost as much as the engine. But you wouldn't go 6 years without changing your engine oil would you?
Why do the manufacturers tell you it's "filled for life" if it isn't?
Well that's because your manufacturer knows your gearbox with absolutely zero maintenance is incredibly unlikely to fail during your warranty period. They can sell you a car offering low service costs (essential for the lease customers who purchase the majority of brand new cars in the UK.
UK vs USA
Our american counterparts run 99% automatic transmissions and they have a far lower fail rate even on much higher mileages on the same gearboxes. The reason is in the culture of the technicians working on your vehicle. A technician state side wouldn't dream about letting a car go 100k miles without flushing the transmission!
Most garages will not carry out automatic gearbox servicing. They have neither the skills nor the inclination to carry out this level of work.
Boring science bit
At a chemical level, three main processes:
Oxidation is the big one. Above ~90°C, base oil hydrocarbons react with dissolved oxygen to form peroxides, then aldehydes, ketones, and carboxylic acids. Rule of thumb: every 10°C above ~80°C roughly halves oxidative life (Arrhenius kinetics). The acids polymerise into varnish and sludge — that's the brown lacquer you see on mechatronic units and valve bodies in cooked DSG/ZF boxes. Varnish on solenoid spools is why an overheated 8HP shifts erratically even with fresh fluid: the acids are gone but the deposits aren't.
Additive depletion. The additive pack degrades faster than the base oil. Friction modifiers in ATF/DCT fluid are sacrificial — as they deplete, clutch friction coefficients shift and you get shudder (classic torque converter lock-up judder). ZDDP anti-wear breaks down thermally and drops out. Antioxidants (phenolics, aminics) are consumed first by design; once they're exhausted, base oil oxidation accelerates sharply — degradation isn't linear, it's a knee curve. An oil at 80% additive life looks fine on a sample; at 20% it falls off a cliff.
Viscosity change and shear. Thermal cycling plus mechanical shear in gear meshes cleaves VI improver polymers, so viscosity drops out of grade — thinner film, more boundary contact, more wear metal, and the iron/copper particles catalyse further oxidation. It's self-accelerating. Conversely, heavy oxidation eventually thickens the oil as polymerised products build up, so a badly degraded fluid can shear thin first, then oxidise thick.
Practical markers: darkening and burnt smell (oxidised friction material plus varnish precursors), TAN rising, viscosity out of spec either direction. In wet-clutch applications the friction lining itself contributes — glazed/burnt clutch material sheds carbon into the fluid, which is why burnt DSG fluid is black rather than just dark.
This is the core argument against "lifetime fill" — the fluid's fine at 40k, but the antioxidant reserve is largely consumed, and the second 40k does disproportionate damage.