I'm also using Castrol Syntec in my 328i.
I ran 5w30 in summer with no noise,usage etc.
I use it because of the recomendation by BMW.
I will be using the 10w30 or a 50/50 mix of
10w30 and 5w50 this summer though. I understand
that A-3 rated oils like BMW High Performance
5w30 and Amsoil are thicker in weight(near 40 wt)
I might just go with the 5w50. I only put about
8000 km (5000 mi) in a year. I was thinking
about Mobil 1 but all the negative things I keep seeing on this site about start-up noise,high iron etc. just doesn't appeal to me. Who cares
about pour point, cold cranking etc. I care about the additive package, TBN, less wear in
my BMW. Castrol is a very good oil using 99.9% pure hydro treated/cracked base with esters that
bond to engine parts and has been silky smooth
in my engine.