Synthetic oils

I've been using Amsoil synthetics in everything I have for years - 1975 Dart Sport race car, 2004 Ram 3500 diesel dually and 2005 Dodge Dakota. Gotten all favorable comments from my race engine, trans and differential builders concerning wear. Basically they said to keep using whatever I'm using! I've seen improvements in operating temps, HP, ET and mileage.
But not all synthetics are created equal. Needless to say, Amsoil is in the better group.

Amsoil info and source:
http://www.thelubepage.com/

As far as oil viscosity, I'd stay with factory specs or slightly thinner.