I'm not "afraid to service a car properly". For me, proper transmission fluid changes have always been drain-and-fills. That has always worked for me since I obtained my first car in the early 1970s....

And the ZF transmission is indeed quirky. Like it or not, that's part of the issue here....

 

Update on the occasional quick "whirring" noise in our ZF during shifts:

It's been several weeks since I've heard the noise, and I've driven the car more than my wife has since she returned from one of her Florida trips with it on September 24. The ZF seems to be back to normal. I reported that to ZF tech support in Chicago yesterday. Their current advice is to just keep driving it without putting in the additive. I asked if this situation has been reported to them previously. They said no, in all other cases they have been made aware of, once the noise begins to occur it continues to occur periodically until the additive is applied....

So maybe our ZF is just in a lull period. But I'll follow their advice. No additive unless the noise returns....

 

Not a new thread but interesting insight. Tx