DO NOT forget driving habits play the biggest role in oil change intervals.
You could put 20k highway miles on synthetic and still have it be passable.
However the same oil under aggressive driving & city driving may be at the passable limit at only 5k miles.
Stop and go & high rpm's are the hardest on oil.
With that said; I change my oil 3 times a year:
Once after winter (5 mos/4k), once in the summer (3.5mos/4k), and once before winter (3.5mos/4k)
Since I don't put a ton of miles on my car it's more a time change deal. If I did rack up more miles I would change every 5k.
The Duratec is very harsh on motor oil!
For $18, Blackstone Labs is a good service to use. A good way to keep tabs on your engine and set your change intervals.
