After living and breathing and doing and yes even standing up for extended oil change intervals, I really think one of the most critical things in an otherwise healthy car, is driving "style".
If you do a good flush, run your choice of synthetic (not sure why you excluded Amsoil) for a short "flush" period say 3K - 4K miles. Drain and change filters, refill - Then start the car, get on the highway. You could drive for 25K+ miles stoping only to eat, sleep and do potty..in other words of all the lousy oil ads - short trips are MURDERon oil.
I live in Lynden and am forced to occaisonally negotiate Seattle from time to time (what now - 3rd or 4th worst traffic??) - that crap and just driving 1 mile to work....well I wouldn't expect any oil to be happy 10K+....
That said your driving style, car, oil and then go say 5000 miles with those oils and see how they hold up in analysis if good push for 7.5K, etc....