I think the reason why people say Mobil 1 is superior is because of advertising, and Mobil's reputation. Maybe Syntec use to be crummy, or average, but i'm not so sure anymore.
I think Castrol has been constantly improving their oils since going the Grp III = synthetic days. I imagine one could get 200k from any car using non GC Syntec provided they weren't in a sub zero atmosphere. Sub zero is PAO territory but they don't have a rock solid lock on this arena any more. I have freezer tested all kinds of oil (we're talking about as non professional as one can get)and Syntec 5w30 was the only one to do poorly. Oils that we have identified here as Grp III/IV blends have passed the freezer test with flying colors! Here is a short list of oils that appear to flow as well as M1 5w30: BMW High Performance Synthetic Oil (5w30), Castrol Syntec 5w40 AC, Castrol Syntec 5w50(!) and Castrol Syntec 5w20.
Let me close with a simple statement: forget the Grp III, IV,V argument and let's just say that all motor oils are getting pretty darn good - just pick the one you feel your engine likes and go with it. Pick an OCI you're comfortable with and let 'er rip!
Sometimes I wonder about the poor sap that just has Jiffy Lube change his oil without any consideration of the matter and manages to get 200k out of an engine without the first AutoRx flush, using Fram filters. We're oil heads alright, and we pour over UOA's as a sport and I imagine all of us have cars that wouldn't dream of an oil related failure, but there are people that, outside of regular oil changes, don't give lubrication a second thought! Cheers! DV