Definitely, 100%, no doubt, go with the ATF as specified by Ford and Borg-Warner. And no additives.
I haven't seen it yet in this thread, but it never fails, someone will chime in and say "it's a gear box, you gotta put gear lube in it". NO. The World Class T5 was engineered to use ATF, so whatever brand you choose, use ATF. Don't let anyone out there think they know more than the engineers who designed this transmission. As an engineer who designs consumer products, let me just tell you it's just silly when people assume they know more about the transmission than the guy who engineered it.
That being said, the T5 it only takes a couple quarts, and it's not like you change it every day, so spend a few extra bucks and buy the best synthetic ATF you can find. Which is best in this application is largely based on opinion, but I've tried Amsoil and I've also run conventional non-synthetic ATF. I've never had a lubrication-related problem. Let's be honest, most T5's that do fail will break, not wear out. I'm on my third T5 and it's not because I'm a high-school powershifter, or that I don't know how to drive stick. I've been driving stick shift for 30 years. The sad truth is, this transmission was rated by Borg-Warner (now Tremec) for 280 lb-ft of torque... while the 5.0 in the 1987-up Mustangs was rated at 300 lb-ft. And that's before any modifications, of which most of us have done at least a few. So if driven enthusiastically, your T5 may eventually break; and there's nothing any synthetic or magic additives will do to prevent that.
If you want it to last forever, drive like a grown-up, don't powershift, don't put a set of cheater slicks on and go hit your local drag strip, and for cheap insurance, change the ATF in it once a year. On the other hand if you're willing to replace it every now and then, put in any name-brand ATF, drive it like your life depends on winning every stoplight-challenge, and have fun!