Consider this an incomplete thread for the time being.
A picture of the tubular strainer (as PSA call it) filter that is accessed from under the car. ZF's number was 0501-311-497.
Filter construction is two layers of fine metal gauze with about 3mm of airspace between. Natural wear and tear in the transmission will result in particles blocking this filter, eventually causing a drop inline pressure. This could impact on shift quality and timing.
Beware that many aftermarket filters don't come with the extra O-ring. The Beck-Arnley unit pictured is Taiwanese, and seems decently made. I cannot say where ZF's own filters are made, if they still offer them.
Wildebeest's comments elsewhere about ZF castings, are a good caution. I have never had an issue with clean threads but they are very precisely cut with a full thread profile, and would thus not respond well to abuse with dirt, crossing or overtightening.