We've discussed this in the past a few times so you might want to do a search and see what's been said. The gist of it is that Fiat claim it should be a
non-EP gear oil. Fiat's version is the Tutela ZC90 oil and it should still be available from most Fiat garages or people like
Ricambio (have a look at
http://www.ricambio.co.uk/index.php?route=product/product&keyword=zc90&product_id=453). Because non-EP gear oils were not widely available in the UK, Fiat UK recommended using engine oil in their gearboxes. You can use a straight 30W engine oil (a 30W engine oil has the same viscosity as a 90 gear oil) or use a multigrade engine oil like 20-40 (I would avoid the very thin 0-40 modern engine oils). A lot of people have used 90EP gear oil in their gearboxes for years and have not had problems but why take the risk.