The brief
The 520i had crossed 100,000 km on its original transmission fluid, and it had started to show: slipping, sluggish responses, and the odd unusual sound during gear shifts, with the changes not as crisp as they used to be. He brought it in for the service. The ATF lubricates and cools the gears and clutches and carries the hydraulic pressure that shifts them. Heat and friction degrade it over time, so it loses its ability to protect and to shift cleanly, which is the slipping and the rough changes. On these BMWs the transmission pan is a unit with the filter built into it, so the proper service replaces the pan and filter together along with fresh fluid, rather than just topping it up. Done at the right interval, it keeps the box healthy and the shifts crisp.
The diagnosis
A check of the fluid showed it dark and tired, well past its best, which fit the slipping and the rough shifts. The transmission itself was sound, no faults stored, so this was a service rather than a major repair, just an overdue fluid change with a box that hadn't been damaged by it yet. So the call was the full job, drop the pan, new pan-and-filter unit, fresh ATF to the correct spec and level, rather than another delay that would only let the fluid degrade further.
The work
The transmission pan was dropped, the old fluid drained, and the old pan-and-filter unit removed. A new genuine BMW-spec transmission pan and filter went on, the bolts torqued to spec, and the box refilled with the correct ATF to the proper level following the fill procedure, with the fluid temperature checked so the level was set right. A road test followed to confirm the slipping and the rough shifts were gone and there was nothing leaking.
The outcome
Smooth, crisp shifts, no slipping, fresh fluid at the correct level, and no leaks. The 520i went home with the transmission back to spec and good for the next interval. ATF is a service item, and a box run on degraded fluid wears harder and shifts worse, so doing the pan, filter and fluid properly protected the transmission and put the shifting right.