we actualy just went through this with my cossie not to long back, had tabetha and I doing a bit research. Our T5s are WC T5s and use fibre or almost carboard components on the syncros, the ATF stops these parts from being damaged or prematurely worn out.
I was running standard gear oil in my tranny to start out with but found it shifted horribly.. once I swapped it out for ATF the difference was night and day.. So smooth even when it was cold!
What I ended up using was Delco Dex 6, it is compatible with Dex 2 and mercon, and what we use at GM in all the WC T5 cars.