The only gear oil which was ever approved by Fiat (please reference the Service Manual) is called ZC 90 which is an SAE 80W-90, API GL 3 oil. It is a mineral, none EP (extreme pressure) oil.
EP, GL-4 and GL-5 oils usually contain additives which allow for better pressure load protection. However these additives might cause chemical reactions on the non-ferrous parts in the box, e.g. synchro's which in return has adverse effects on service life and shift comfort.
On the other hand the synchro's are save with the GL-1. But this oil does not provide sufficient pressure load protection which can cause wear on the gears and synchro's.
Check this link for more details on the API GL specs:
http://www.lubrizol.com/AutomotiveGearOil/GL5.asp
Since Fiat left the North American market I could not find this oil anywhere and ended up having a buddy bring me some back from Germany
Over there it is readily available at every Fiat dealer. It is marketed under the label of FL Selina. The oil is called Tutela ZC 90