just dug out the manual for the car, for the 2.5V6 Engine oil quantity is 5.9 litres which includes the oil filter.
For oil it says
Selenia 20K SAE 10W40 ACEA A3-96, API SJ
* for sports type driving Alfa Romeo recommends Selenia Racing 10W60 entirely synthetic oil
it also mentions
warning Do not top up with a different engine oil than the already existing one
but maybe peoples experience on here suggests the 10W60 as the norm, as they have more experience over the years with the engine than Alfa did when they made it.