I would recommend the ZFs get serviced every 60k (ZF themselves recommends every 100k km, which is pretty close to 60k miles).
You can do it a couple different ways. Flush it with a machine, hooked up to the cooler pipes, or drop the pan and replacing the filter.
I prefer the latter method, because the later 6sp and all 8sp ZF transmissions do not have the filter attached with screws to the valve body like the 5sp and early 6sp did. So, in some cases, the filter can drop down due to the little orange seal that holds it up in the transmission shrinking/hardening over time. While the filter cannot completely fall out, at it will land on the pan first, it can cause a less-than-ideal ability to do its job of being the fluid pickup, and cause the pump to suck air. This causes over time an aeration of the fluid, which of course is not good.
If you take the pan off, and the filter falls out with it, you know it had come loose. And was just "sitting" in the bore. You'll have to get up in there and remove the old seal. The new one will be on the new filter already. And get a new gasket, and do NOT overtorque the screws, or it'll bend the pan and it will leak forever and require a new pan.
The drain plug seal and check/fill plug seal are not available by themselves, they are part of the plug, so I recommend getting new ones... especially if the check/fill plug is the plastic one. You for sure won't want to reuse that and cause a leak.
The fluid is not cheap, but there are some aftermarket alternatives that are a direct replacement for the ZF 6/8HP juice. You'll need a device to push the fluid back in, same as most other modern autoboxes.
There is a forward brace that has to come off to access the pan, as well as the rear transmission mount assembly (which requires the rear of the trans to be supported during service with a suitable jack).
The car will need to be level for checking/filling.
Easy job, not a big deal really.