Hey.
Generally Mobil 1 Extended Performance or Amsoil is a good synthetic to run for an extended drain.
I run my 3.7L V6 SOHC engine about 12k miles a year, and change my Mobil 1 Extended Performance every october...
My Father has A 03 Ranger 4.0 V6, and he does his Mobil 1 Extended Performance every 10,000 miles. He's got 105,000 on the engine currently, does not seem to burn any oil through the entire 10,000 miles. I change the oil for him, and use a regular supertech/purolator/stp filter, or whatever is available each time.
I dont have any Oil analysis results for the 4.0 with the 10,000 miles oil change interval, my father does not care enough to have one done.
However, with Mobil 1 Extended Performance, or Amsoil, you can run a 12k-15k change and should not have to worry about it.
But, if you want to know every detail, simply run one of these oils for 10,000 miles, get an oil analysis done, and then you will be able to determine whether or not you should run the oil so long. IMO, if you want to switch to synthetic for extended drains, its not worth doing unless your at least getting 10k out of the change. Others might disagree though.
Someone on here might have results for the 4.0...
There are people with alot more experience on here then me though. I am not as a fanatic

. I simply want to change my oil once a year.
Good Luck!