We have an old seasoned builder that builds T-5s for our shop in exchange for cores, and he swears by 10w40 motor oil, and that is all he wants in his transmissions. They shift way smooth.
We've used ATF in T-5 with brass blockers, although a lot of people here berate me for our use of it.
In later model units, I believe you have to use synchromesh, as (I believe) ATF deteriorates the composite bonded blockers in them. Not sure about the powdered metal ones, but they probably don't like it either.
We used synthetic, and people said they felt notchy, and went back to motor oil. Sychromesh was fine as well.