I didn't take the time to read the entire, but I'll add my thoughts on the matter anyway.
I used to run exactly 3000 miles and change the oil. If I was over a bit, I'd freak out and think my engine was going to blow up. Guess I was pretty naive at the time.
When I got my jeep and decided to switch all maintenance / most repairs to ME, I started reading up on oil. My 4.0 is an "old" designed engine with not-as-tight tolerances as modern engines. Anyway, the common theme was that it is safe to run 5000 on synthetic on that engine.
My 2011 Focus' owners manual suggested 7500 miles OCI. At first, I wasn't entirely sure whether or not that would be a good idea. When looking at used oil analysis for this engine, I think 7500 on a synthetic would be entirely reasonable. Maybe even 10!
There's a few reasons why it's able to go 7500-10k miles. First is, it has a larger oil capacity per liter of engine than my Jeep. It's oil capacity is 5 quarts, my Jeep's is 6 ... it has half the engine displacement. Second, as with any "modern" non DI engine, it has tight tolerances and isn't hard on oil. Until recently, I had been doing it at 5000 since I qualified as "severe service".
Since my last oil change, I've had a ton of highway driving, I'm going to try to extend it out to 6500 this change. I need to keep it within owners manual specs for warranty purposes.
Once I get out of warranty, I'm going to do two oil changes a year. One in November switching to 0w-20 and one in may/june switching to 0w-30 - it should be about 6000-8000 per change. Oil filters will be changed haflway through the interval.
Now, DI engines may be a bit different. Since they're new, the affects of using oil longer aren't entirely known. They do tend to be a bit harder on oil and have more fuel dillution. If I had a DI engine and was going to do an extended interval, I'd do a used oil analysis at 6000 to see how the oil is holding up.
2011 Focus SE 5 Speed 36k - Blue Flame Metallic - Added Rear Swaybar
2000 Jeep Cherokee Sport 115k - Lifted with mud tires.