Change the fluid. The latest info from ZF 10/2019: ZF automatic transmissions are generally filled with lifetime oil. However, due to the many factors influencing the service life of transmissions in individual operation, ZF recommends an oil change after 150,000 km for its transmissions. In operating conditions with high temperatures and loads, or with unknown vehicle use in the past, it can make sense to change the transmission oil at shorter intervals. https://aftermarket.zf.com/remotemed...fluid-8-it.pdf

The fluid BMW uses is also known as lifetime fluid (not forever fluid). This fluid is less prone to chemical and viscosity break down compared to conventional oil and suitable for up to 100,000 miles.