Castrol 5/30 Synth here.
Advantages of Synth;
1. most are 35% synth + 35% dyno + 30% synth additives, so you are using more synthetic compounds (less dyno oil) so it is more "eco" than regular oil.
2. You can extend your oil change to 20k miles, so price ratio is almost the same as dyno oil.
3. Better lubrication properties
4. Better fuel economy
I know it is a pain to pay that much more, but in reality in the long run it is better for your car, environment and wallet.
Rob Hernandez, AFM co-Founder.