I used to use Valvoline 10w-40 Full Synthetic (just because it was the best option at my local store).
Now I'm using Amsoil Synthetic 10w-40 Scooter Oil
Amsoil vs. Mobil 1
http://www.ultimatesyntheticoil.com/..._vs_mobil1.htm
Amsoil vs. Royal Purple
http://www.enhancedsyntheticoil.com/..._vs_AMSOIL.htm