You should always follow the manufacturers spec for oil.
Ford T5 EB Fowlcan is Dex III.
When I first installed my big bore M21, I used (Penrite SIN75) not realising it was synthetic. I had second gear crunches straight away but all the other gears were fine. I dropped out and discarded $50 worth of oil to use a mineral (85) based alternative as suggested by other torana nuts. (Although my step brother from Mobil stated the sync was not the issue). This was after only a couple of hundred kilometers of travel from new.
The end result was I still had the second gear issue just as bad.
As some off you know the box crapped itself after 1400k and has been rebuilt. The second gear setup was inspected but not replaced at the time as nothing significant was noticed. Once the box was put back in service, the second gear gremlin was back to play. Grrr.
Yes I am still using the Penrite SIN75, which has obviously nothing to do with the second gear gremlin from the above experiment findings.
I am finding it leaks out of the rear main seal though. Which pointed out in some of the above posts is a problem with ATF.
Eventually it will leak no more as the level drops.