Although I am an exception to the rule (I drive 97% highway, 25,000+ miles/year), I run synthetic with a minimum oil change interval of 10,000 miles.
I ran a 1995 neon hard for 140,000 miles on full synthetic (Amsoil 0W30, 30,000 change interval/filter change-top of every 10K) and it ran like a top the day I traded it in on my cavalier in December 2001.
My Cavalier made the synthetic switch at 1,900 miles first to Amsoil, then to Red Line (the Amsoil dealer quit on me). After a while I decided to switch to Mobile 1. I'm currently running 0W20 Mobile 1 with changes @ 15,000 mi. The cavalier is closing on 83,000 miles and seems to be pulling harder than it ever has, especially at high rpms (DOHC engine, 6,200 redline) compared to when it was new.
Even the factory manual says dino oil is good for 7,500 under my driving conditions, so I see no problem running 'regular' Mobil 1 synthetic 15,000.
BTW, I wrote Mobil a while back and asked then about extended drain intervals. They told me their oil has been proven to 25,000 miles, "but" they said they (prefer to) warranty their oil to the max mileage listed in the owner's manual for changes. The really funny part is, less than a month later, they "introduced" their 7.5K and 15K oils.
Kind of makes you wonder if they had been hiding on the band wagon of "3 months or 3,000 miles" just for sales profit, then decided to join the "extended drain interval," "Premium Synthetic Crew" once they realized the popularity :whistling :ermm:
This is just my opinion and I have absolutely nothing to back it up, but I think the 15,000 mile "Premium" oil is nothing more than the SuperSyn Mobil 1 repackaged and the 7,500 mile and 'standard' synthetic oils are the "reformulated" ones - their additive packages have been reduced to lower their costs. Otherwise you'd have thought they would have bragged the hell out of them more than just saying "Good for 15,000 miles." Look how big they bragged about being the oil of choice for the military, and the factory fill/required oil for Porsche, Viper, AMG etc....