What the data says
You may have been told the transmission fluid never needs changing. We do not have a BMW source for that wording, so we cannot confirm or dispute it. What our pages do show is owners and shops doing preventive services, listed as around 60,000 to 100,000 miles for the ZF 8-speed and around 50,000 to 60,000 miles for the E9x M3 dual-clutch. Harsh or delayed shifts and shift flare at higher mileage are the symptoms listed. The ZF 8-speed page covers most BMWs from 2012 on with the 8HP45, 8HP50, 8HP70 and 8HP75.
ZF 8HP fluid service
The final fill must be done at the right fluid temperature, so the shop or DIYer needs a scan tool (ISTA or an app like BimmerLink). Ask any shop how they handle that step. The page notes the dealer price is not well covered, and one Canadian dealer quote was for fluid only. See ZF 8HP Transmission Fluid Service.
ZF 6HP: mechatronic sleeve
On the ZF 6-speed in E9x, E60 and E70 cars, transmission fluid dripping near the electrical connector or wicking into the connector is the leaking mechatronic sleeve. The sleeve is a cheap part, labor and fluid make up most of the bill, and it is usually done with a pan, filter and fluid service. See Transmission Mechatronic Sleeve (ZF 6HP).
M3 dual-clutch gearbox
On the E9x M3 with the 7-speed M-DCT, fluid weeping from the bottom pan, side cover or mechatronic sleeve is the reseal signal. A fluid-only service is a different job from a full reseal, which needs the transmission dropped. See E9x M3 DCT Service & Reseal.
Before replacing a transmission
The replacement page says to get a pan drop and diagnosis first, because many "failed" transmissions are a mechatronic or fluid issue, and used units must be synced to the car. See Automatic Transmission Replacement. For other drivetrain fluids see the BMW Differential and Transfer Case Fluid Service (xDrive).