I'd agree that between Mobil 1 and Syntec it's a draw... I'd also pick Mobil for the availabiliy but either is good stuff.
One thing I'd do (maybe passing on a wives-tale?) is after switching to synthetic, for the first change interval or two... swap a fresh filter on at 1000 mi, 2000mi, then drain + new filter at 3000. The second interval, maybe a filter at 1500, and 3000, drain & change at 4500. After that, a filter at 2250 and a change at 4500... I don't think I'd go much beyond 4500 on any oil... but that's just me.
The "wives-tale" being the synthetic oil's detergent properties will "shock & awe" a bunch of sludge loose in it's first couple visits. I guess it depends on how nasty the motor was to begin with... Ideally one would switch over to synthetic oil after the engine has just broken in (5000-7500 or so with 2-3 changes of dino), and sludge buildup wouldn't ever be a real issue.
FWIW I use RotellaT synth 5w40 in my bike, and Dino Castrol GTX in everything else, all at 3000 mile drain & change filter intervals.