Gee, I spoke too soon about how wonderfully my Z-16 is holding up! I'd been thinking how I like a pretty matte look to my tires and others might very well want far more sheen that mine have maintained through the winter...that was one thing, and I was thinking about this thread when I washed my wife's A8 today.
After today's wash (tires washed with diluted wheel cleaner again), the Z-16 is pretty dead. This must've been the last straw that made all the difference. They look OK from a few feet back considering it's still winter in Ohio, but I know most Autopians would be pretty unhappy with them. I'm just gonna let them go until I put the summer wheels/tires back on, but I'm certain that after the *next* wash they're gonna look pretty sorry.
But still, I got through most of a tough winter without redoing them, and that's plenty good enough for me. I believe DavidB hit the nail on the head- let the Z-16 set up for a while before you buff it off. That and getting the tires *truly* clean. By the time I applied my Z-16 they really were clean, no staining of the white cloth I used with the Z-16.