I spent many hours researching this again yesterday, and came up with the same results. There are at least five oils listed as suitable for the TKX. Dex 3, Tremec, Mobil 1, Amsoil, and Synchromesh.
If there is a better oil to use, Im all for it. One thing is for sure, when the transmission is new, during break-in, there's probably more heat and debris than there will ever be again, so if Tremec thinks that Dex 3 can handle that, then I don't know why it couldnt handle daily driving.
Either way, as I said, more info is needed... And, of course now that Tremec is selling thier own oil, I expect this information to be more vague/ hidden than ever before.
Harley Davidson oil is the ONLY oil suitable for use in their engine, transmission, and clutch (yes, Harleys require three separate oils). Well the engine is an easy one, but when you ask, what kind of oil does Harley use in the trans and clutch, the answer is, Harley Oil.
So I guess the answer to this question is Tremec oil?