You misunderstood me. I meant, sometimes, they do not put enough oil and, sometimes, they do not tighten the oil filter enough (and it leaks there).
As for the higher mileage oil for performance and power, I'll let those who have more experience with those answer. I have a stock Duratec and have not used "high mileage" synthetic oil.
Ford changed the specs recently to 5W-20 for fuel economy improvement (how noticeable, I don't know). I stayed with 5W-30.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=v6maint&Number=1228688&Forum=v6maint&Words=5w-20&Match=Entire%20Phrase&Searchpage=0&Limit=25&Old=3months&Main=1228662&Search=true#Post1228688
