Stick with the OEM Mercon V ATF. ATF is specced for my 6 speed Camaro SS. Most Vettes of the past 25 yrs use/used ATF in their manual gear boxes. No problem getting a long life out of these with proper fluid changes and proper shifting techniques. Walmart SuperTech Merc V ($4.25/qt) is Ford approved as is the Castrol variant.
If you want the best ATF's then look at Redline D4, Amsoil ATF, Mobil 1 ATF and other top synthetic brands. Those come with better additive packages than some of the generic Mercon V's. Just because a higher end fluid (like Royal Purple Synchromax) gives you better shifting "feel" like butter, doesn't mean it's doing the same good for the transmission. You can get a smoother shift feel, and be ruining your synchronizers in the process...could take tens of thousands of miles though. You won't know that until most of the friction material is gone or blinded. From what I've read of Tremec manual transmissions of the 1993-2006 era, RP synchromax is the last "upgraded" synthetic fluid I would attempt to use.
The Mercon V semi-synthetic fluids like ST, Castrol, Amsoil, and others, still have a fair amount of shear vs the full synthetics. The conventional ATF's in a manual shift transmission can be sheared down within 8K-10K miles. The Merc V might get twice that life. And the synthetics maybe another doubling in life.
