The correct FIAT oil is their own "ZC 90"
There have been a number of discussions in the past about what (else) is suitable, as Haynes suggests you can use engine oil.
The FIAT stuff is GL1 (very early specification, with little in the way of additives, apart from something for "anti wear").
According to a site, which I can't find the URL for at the moment,
might be here, or one of the links off, there are different viscosity numbers for the same grades of oil depending on if it is rated as engine oil, or gear oil:
gear oil numbers are higher - 90 is around the 20(W50) mark as an engine oil.
You don't want an oil which is designed for hypoid (back axles) use as the sulphur additives can eat bronze parts in some gearboxes[xx(]
You don't want the latest specification (GL4 or 5) gear oil as the antislip additives will stop your synchromesh from working properly: it needs some friction to get the bits to run at the same speed and not get a crunch (especially into third).
So I suppose the short answer is:
FIAT stuff is the best for the job, but (relatively) expensive in the UK.
Some 20W50 engine oil would probably not do any harm.
You could go to <s>"Wilko"</s> Wilkinsons Comma at 2.09 UKP litre - but it's GL4 and buy the stuff they sell in 1 litre tubs.
Or, you could go to one of the "Countywide" Farmers shops and get 4 litres of Q8 branded gear oil but it's GL5, and some engine oil too, for about 10 UKP a tub.
will check with local factor tomorrow. To be honest I use fully synthetic fiat stuff and wouldn't put in anything else.
