ok i know you are a wealth of knowledge Hissin. I respect you also. Here though is my reason. When the t-5 was designed synthetics were not around. I have seen truck tranny's (not the t-5 tranny though) have premature syncro failure due to using synthetic oils because of the fact they are slipperier then regular oil. This may not be the case with t-5 tranny's as i mentioned but I think that using the reccomended oil that ford says to use would be the best.
The reason you don't use anything other than Mercon is primarily because of the incompatability between other types of fluid and the seals and needle bearings (if I remember correctly, they were the culprit) within a T-5. If you have a tranny that's been rebuilt by say... "Speed Shop X" for use with other types of fluids then press on with pride. Otherwise... stick with the manufacturer recommended fluid. It's what the tranny was designed to run with.