I have used Syntec for the past 75,000 miles and plan on keep using it and everything is good on my current 02 GT. I used regular Castrol in my previous Mustang 1993 4-cylinder, 150,000+ miles and it never burned oil. If you change your oil regularly and you have a stock engine, i.e. not a race engine, I doubt it really makes a difference between the two. But that is just my opinion which is not supported by any facts.
I agree with those who voted for Amsoil, however it is too much of a pain to get where I live so I have always religously used mobil 1 5w-30 in all of my mustangs. Although as money gets tighter I have been considering switching to syntec to save cost...
I like to baby my stang as much as the next guy, but I just thought of something. I think using the really "high end" oils like royal purple and amsoil is really a waste of money, unless you are really putting out a lot of power, above say 500Hp or done extensive work to your engine. Our engines are not that special where they need special oil. We have the same engine as crown vics, and I know that most crown vic ownes don't use these special oils. Heck, I doubt most of them use synthetic oil, and look who drives these types of cars, cops and cab drivers. I would say they are harder on the cars than most of stang drivers and those engines last for 200,000 miles plus. Other examples are the other Ford engines. The engines in the SUV's last for 200,000 miles plus and most of the owners probably places like Jiffy Lube to change their oil, and I doubt the SUV owners use synthetic. One last point, stop and go driving is a lot harder on engines than going down track once a month. I say all this, but I still wouldn't use anything but synthetic, so I guess I am a hypocrite, oh well.