unless you race or track the car, or abuse it, near anything "good" ( meaning ACEA A3 rated) is just fine. Unless the oil is over 100 degc and turning at hgih rpms continuously, AND under heavy load, any old 5w30 is probably fine - if its clean, free of acid, and free of moisture.
(anyone really disagree?)
If you only drive warm weather, thick oil is certainly beneficial - although it eats some fuel economy.
The castrol syntec 20w50 is ACEA A3/B4, porsche approved, and JPF approved :-) I think its excellent. Typically i've been running Syntec 10w40 since its available, and i do drive my car all winter.
I think we tend to get into theoretical debates over what is "best", when in reality we can feel comfortable with "good enough" - and keep it fresh, clean, and dry.
The two things i would avoid are:
a) non ACEA A3 rated oils
b) any oil - and this applies to some of the most hgih tech super oils now being introduced -- that has a very wide viscosity range - 5w50 for example. They cannot be made without additives that result in shear down and sludge under extreme use. And if the use isnt extreme, who needs 'em? Can't have it both ways IMNSHO.
I also like ELF/Total full synthetic, obviously AMSOIL and REDLINE are very good oils, although i can't tolerate the religion that goes along with them.
Grant