If a high quality synthetic oil is used, why not?
I'll give you a couple of reasons. I grant you there are some pros on this board who can perhaps go 10,000 miles on an oil change but they are experts and have a very clear understanding of what that entails. They must do the groundwork in preparing for a long oil change interval by having at least one oil analysis performed and preferably two. They must also know that if their driving habits or environmental conditions change during the course of that long oil change interval, they must reevaluate whether they should change their oil earlier than 10k. (dusty conditions, etc.)
Most people don't do oil analysis and don't have a clue what their TBN or wear metals are. The 2.0 Duratec is an excellent engine but any direct injected engine could also be subject to fuel dilution of the oil. I would say that I have not seen a major problem with that but without an oil analysis, how do you know for sure? I still stand by my recommendation that the typical MK3 owner who does not perform a Blackstone UOA should follow Ford's recommended oil change interval for their vehicle even if they are running top shelf synthetic.
Then there is the matter of the filter. Even if the oil is able to hold up with minimal wear metals and a decent reserve alkalinity (TBN), the filter may plug up before 10,000 miles and open the bypass valve. Not good. I run a Motorcraft FL-400S. It is slightly longer than the OEM FL-910S and contains a little more filtration media. Oil filters have a finite capacity (the amount of contaminants an oil filter can hold before it becomes obstructed and causes the bypass valve to open) The longer the oil filter is in service, the more debris it is likely to collect in the filtration media. If the bypass valve opens, unfiltered oil is allowed to pass to the crankshaft and engine bearings as in the diagram below.
Ford recommended oil change schedule:
Typical driving habits: Every 7,500 miles or every six months, whichever comes first.
Less typical driving habits: Every 5,000 miles or every six months, whichever comes first if:
Towing or carrying heavy loads
Idling extensively or driving at low speeds for long distances
Driving in dusty conditions
Driving off-road frequently
Using E85 fuel more than 50% of the time
Diagram: Oil filter bypass valve closed/open