It may be ZF did not recommend maintenance, although I doubt it however today they do:
ZF Parts | How to change oil
ZF automatic transmissions are filled with specially developed oils. At very high operating temperatures, the oil ages faster than under normal conditions. Depending on the driving style (e.g., many trips at high speeds or sporty driving), ZF recommends a transmission oil change every 80 000 to 12 000 km, or after 8 years at the latest.
Also, for anyone else doing this, the o ring & pan gasket are must change items.
I am a penny pincher and looked high & low for the "right" path as well as right parts. I bought Castrol and was convinced by members here to go ZF & returned the Castrol.
I found the best price for the filter kit at CTST and got the Lifeguard 5 from different sources finding just over 14.00 plus shipping the best price.
BMW, Audi Volkswagen etc all buy the ZF parts from ZF.