The brief
The 540i was getting on in mileage and the owner wanted the automatic transmission fluid done. That's a smart call, the gearbox on these is a sealed-for-life unit on paper, but in practice the fluid does age, and changing it at higher mileage keeps the shifts crisp, helps the gearbox run cooler, and heads off the expensive problems before they start. The job is more than draining and refilling. On this ZF eight-speed the filter is built into the pan, so a proper service means dropping the pan, replacing the pan-and-filter assembly with a fresh gasket, refilling with the exact ZF-spec fluid, and then setting the level with the gearbox at the right temperature, because on these it's a fill-to-temperature procedure, not a dipstick. Done right, it's the cheapest insurance there is on a transmission.
The diagnosis
A check confirmed the gearbox was shifting fine, no slipping, no fault codes, just fluid that had done its miles and was due. The pan and filter had never been off. So it was a straight fluid-and-filter service: drop the pan, fit the new pan-and-filter assembly, refill with fresh ZF fluid, and set the level properly.
The work
The old transmission fluid was drained, the pan dropped and the pan-and-filter assembly replaced with a new genuine ZF unit and a fresh gasket, the magnets cleaned off as it went. The gearbox was refilled with the exact ZF-spec fluid, then brought up to the specified temperature and the level set precisely the way the procedure calls for, with the fill plug torqued to spec. A road test confirmed clean, crisp shifts through the range with no flare and no hunting.
The outcome
Crisp shifts, smooth pull-away, no flare between gears, and the gearbox running cooler on fresh fluid. The 540i went home with the transmission serviced properly. Fluid that's done its miles loses its edge and lets the gearbox run hotter, so changing it, the filter and the gasket and setting the level right is the proactive job that keeps a good gearbox a good gearbox.