I would wait and see. Second guessing because it calls for Z1, or second guessing because there is actually a noticeable difference in shift quality? That would be the question. A lot of folks just get paranoid when they read all the posts that say...stick with OEM, stick with OEM. Which is nonsense.
I just changed out 3 quarts of 7.2 quarts a couple of days ago. The 7.2 quarts I had in there had 13k miles over 2 years. The car was a little sluggish on shifts (but its nothing major) and after i changed 3 of it to Mobil 1..it's a little more crisp. Im sure whether i put Z1 or Mobil 1 Synthetic ATF...it would improve. I plan to do another 3 quarts drain and fill in the coming week. So far so good.
Yes, it has been a big improvement for me using Amsoil ATF.
I do plan on going to Mobil 1 in the future though. Unless I can be swayed to go back to Amsoil products. But, that is a different thread for a different time.
anyone correct me if I am wrong but - I dont think you will be able to find any publicly published or official "specs" for Z1. Nor would you be able to find if its dino or blend or synthetic.
Best you could do is refer to VOA and UOA to get an idea of the viscosity and flashpoint and possibly add pack.