I wasn't too aware of Agip and Selenia sharing the same base so you learn something every day.
I know that a 10w-60 from one manufacturer will be the same thickness as a 10w-60 from another though. Maybe I should have re-phrased my comment on why did they make Selenia racing so thick. What I should have said was why did they make Selenia racing 10w-60.
Although both the 10w-40 and 10w-60 are the same viscosity when cold, obviously the 10w-60 will thin less whan hot.
It may appear that this will give the best protection, but that isn't nessesarily the case. An oil will give its optimum lubrication and protection at around SAE 30. As we know all oils thin as they get hot, so as the oil warms up there will be temperature of the oil where it behaves as an SAE 30.
The temperature where this occurs will be higher on the thicker oil. So basically a thicker oil may never get up to correct operating temperature, epecially in car that is used for road use, in a cool climate. Many Alfa's will also have oil coolers.
If the oil is not getting to temperature it will cause more drag, not circulate correctly, create cavitation in bearings at higher engine speeds which can increase wear.
Many Alfa's (mostly JTS's) that were specified with a 10w-60 oil have suffered cam wear due to poor oil circulation.
Unless a car is used in an extreme climate and driven hard I really believe using a 10w-60 oil will do more harm that good in the long term, as the oil will not get to its optimum operating temperature.
My Fiat coupe (producing over 125bhp per litre) rarely went over 60ºC on the oil temp gauge, and that was after giving it a severe caning. I very much doubt an Alfa in probably a milder state of tune would even get that hot.
I ran the Fiat on 5w-40 fully synthetic, and it gave excellent oil pressure at tickover straight after a thrash despite being no spring chicken.
At the end of the day it is up to the owner what they use in their cars but thats my 2p.