I know a lot of people have already discussed this, but I ran across an interesting pdf file (see below) from Conoco Phillips.
It shows a comparison between different grades of oil bases. Most motor oils fall into either Group II or Group III base oils. Some of these oils are classified as "synthetic" even though they are just highly refined mineral-based oils. Group IV base oils are PAO-based (polyalphaolefin). This includes Amsoil and Mobil 1.
The pages I found most interesting were page 5 (showing cost/performance) page 12 (viscosity index) and page 19 (shows correlation between base oil and fuel economy).
Some of this is kind of technical stuff and I'm not even pretending that I understand it all, but I still found it interesting...