I've tried both Z8 and Z6. I prefer Z6.
Z6 doesn't seem to streak as easily either.
I had a few bottles of Z8, which I reserved for just shows. But once they were empty, I resorted back to Z6. Its very easy to use, and the results are just as good.
As for the tires, I use Armor-All Extreme Tire Shine (spray, not gel).
Here is a pic. after using Z6, and the tire shine mentioned;