Car Manufacturers normally define what oils are used in their engines during the design phase of the engines. Changing or using different oils is up to the owner of the car and what he/she feels is the best in their nominal opinion. Examples below.
Alfa Romeo engines prefer Selenia
Renault engines prefer Elf
Citroen engines prefer Total
Ferrari and Maserati engines prefer Agip
Lotus engines prefer Castrol
VW/Audi engines prefer their own spec oil
The list goes on and on.
In my opinion a 16V Alfa engine should use 10w/40. A 8V engine should use 15w/40 and a 24V engine should use 5w/40. From my expirence using alternate oils is not viable. 6000mile oil changes are a joke anyway, why do the alfa romeo dealers in Italy do oil changes every 5000Km. which is about 3000miles.
20% of the detergents and 40% of the oil properties are lost in the 1st 2500miles anyway. Which makes sense doing oil changes every 3000miles anyway. Your oil should only discolour above 3500 miles, if not then the properties of oil of COOLING, LUBRICATION and FILTRATION are not being met. Hence piston ring blow past and oil getting burn't and engine getting too hot.