DEX 2 or 3 is fine.
The early cossie T5 boxes had a material on the ends of the selector forks called pads that were made of paper, well compressed card, ATF disintergrated these very quickly.
ATF as it is so slippery, makes for a smoother change, but synchros need FRICTION to do thier job, this they get with gear oil, gear oil is also better for shear load, that atf was never desinged for.

tabetha