There are as you know over 100 versions of the T5, the T5 used in our cossie was indeed WC, however when they were made initially they had the pads on the fingers of the selector forks made from a type of compressed card, ATF would happily destroy these pads, that is why ford commissioned a special ATF, ie not your regular stuff.
Additionally ATF has very less shear ability but gear oil has it in abundance, another point to note is ATF is VERY slippery(ford even used this as a bodge in press cars to squeeze more mpg), good you'd have thought but not for the synchro rings which actually need FRICTION in order to do their job well.
tabetha