I can't find details for that specific model in English. The other50's/60's singles manuals usually recommend SAE 40 for winter and 50 for summer. I've been running my 57 175 Sport on:
Multigrade low detergent/dispersant engine oil. Suitable for many
pre 1990s classic motorcycles.
Not all the manuals give a capacity and when they do it's often in gms rather than cc. Most work out just under 2 litres though.
What sort of filler has it got. The 57 has a strange horizontal 'tray' beneath the filler cap. The oil level has to be just lapping that in my experience or mine at least tends to smoke. Certainly does if filled right to the bottom of the filler plug thread.
The 175 Rossa Extra may hold more oil though being wet clutch (57 'sport' is dry).
There is this handbook in Italian which doesn't say oil type but gives capacity as 1.8Kg, which allowing for the specific density of oil works out at just under 2 litres.