I put something on similar before about this, my mate sells oil and talked directly to the Castrol labs/technical department about this subject, and the answere is pretty much as Ian has described it.
Older design engines have greater tolerances (ie the machining wasn't as good and the gaps are bigger - thats why just machining and blue printing can give a generous power increase on something old (and crap)like a Pinto engine), hence you need the thicker oil. The Alfa V6 in the 75 and 155 has been around since the late seventies if not before.
Also, unless you are running some highly tuned engine on the road, you are wasting your hard earned cash using a fully synthetic oil, it doesn't need it.
If you change from synthetic to semi or vice versa, give the engine a good flush. It is correct that the residues/detergents can confilct and could cause premature wear in a worst case scenario. As far as possible, stick to the same oil once you've found one you are happy with, try and avoid mixing oils at all, even in small amounts.
I also use Castrol Magnatec in all of my road cars, 16v and V6's, it's plenty good enough. Another mate has a 400bhp turbo Toyota, I get him Castrol Formual RS (but still 10/40) for that because he likes to cane it.