Yes T5's do use automatic trans fluid. The synchro's have organic linings bonded to them similar to the clutch packs in an automatic trans or limited slip rear. If you use regular gear
oil the synchros won't work.
I agree, use Redline, they make a manual trans fluid that works extremely well in T5's. You'll find that the trans never shifted so smooth. My T5 has 150,000 miles on it with one minor rebuild without getting blown up. The secret? Get a shifter with positive stops and don't power shift third gear.