I don't claim to be an expert, But I have owned my 3.0i since 2011 and as it stands its total mileage is short of 37000. With such a low mileage I was a bit disappointed with the ZF gear change, it was very vague and had a rough or harsh feeling when selection was made. My cure was to change the shift detent springs. The latest pins have re profiled nose and the kit of three pins including teflon bushes seals and circlips was around £50.00 from BMW. These pins are located at the top of the box and receive very little lubrication and run virtually dry. This problem is overcome by the teflon bushes allowing easier rise and fall, and the re profiled noses allow the pins smoother transmission from one detent to another. My gearbox is now silky smooth as it should be. Like I say, I am not a Z3 expert, and can only report on my own findings.
this may give food for thought.
Blue skies Chris