"Motorcraft Full Synthetic Manual Transmission Fluid XT-M5-QS" from Germany which is a 75w90 GL-4. It's very popular with the Miata/Mazdaspeed and Focus crowd amongst others. Has an enormous following and positive results. Probably the best GL-4 fluid around.
Just Google it for more info and testimony. It's one of those "ad nauseum" topics that is so well established, forums hate seeing new threads about it.
It has an immediate effect with many people having notchiness problems but it is also a lifetime fluid meaning simply that you don't flush it out after a few thousand miles. You leave it in for good and it keeps getting better over time.
I have it in my Miata 6-speed which shares the same Aisin tranny as the S2000 and never looked back. I just got my Z recently and am going with the Motorcraft in that too. I'm not affiliated with Motorcraft, haha, just swear by it from experience.
Info:
https://www.fcsdchemicalsandlubrican...ission%20Fluid