Mobil 1 used to be a true synthetic, I ran 15W50 Mobil 1 in all Mustangs and friends ran it in there turbo cars as well. It used to be one of the few Group IV oils, now it is a Group III like all the rest of the synthetics. Mobil 1 actually sued Castrol over this years ago because Syntec was not a true synthetic. Ironic now neither are they.
However, Castrol Syntec 0W-30 is a full synthetic. Sometimes called "German" Castrol, it is made in Germany and a dark green color and is VERY good oil.
Does it REALLY matter on the whole Group III or Group IV oil? Not really.
One of the best performing oils you can buy anywhere is the Pennzoil Platinum. Do some research on the Royal Purple and you will see that it is over-rated and is outperformed by many other oils available anywhere for MUCH less.
There is nothing wrong with a dino oil either, as long as it is changed every 3k miles it really won't matter. I have taken apart a 5.0 with 130k on it that lived in Valvoline 10w40 and it was spotless.
I would suggest some reading on
www.bobistheoilguy.com, VERY good info in there.
And for older cars or anything with flat tappet cams, Rotella oil is awesome. One of the few oils out there that has a decent amount of zinc in it. Same with Valvoline "Race" oil and others labeled as that. They are branded race oil due to the zinc content and other additives that are not suppose to be used due to emissions (mostly converter longevity).