Just to add up basing on my own experience:
1.6L in reality I was able to fill in proper garage on flat surface.
C510 gearbox, more likely yours as well is C510.
I tested myself oil 75w90 GL-4 because I wanted something slightly thicker to save my gearbox for longer while driving hundreds of miles without breaks. I found it not good enough in the UK --> brilliant during summer but I experienced hard to shift 1st and 2nd gears in winter. So it would be perfect for Spain for example but not for the UK. Two cars the same so it must be oil then as it wasn't a problem before.
Bought now in my opinion the best one dedicated for C510 and mild/cold climate:
TUTELA TECHNYX 75W85 GL-4 plus. I'll see next winter how it goes.
(Just 75W according to many Alfa Romeo users with C510 gearbox was not good enough, but 75W85 they found perfect, going lower than xxW85, like 75W which really is something like 75W75 may reduce lifespan of gearbox when driving hard/distances/sitting in traffics without much breaks, that's how decision came regarding 75W85.)
Fiat specify Petronas Tutela Gearforce 75W oil, 1.65 litres capacity. It's an API GL4 spec oil. But any quality 75W GL4 transmission oil should do.
