I would suggest the 15W-50 over the 10W-40 because it makes the transmission shift a bit smoother. I use the Triumph oil just to avoid any warranty issues. At the recommended 10K mile change interval, you really aren't talking much per mile (I change mine at 5K cause I'm not cheap like Draggo).
The oil debate continues...
If i'm not mistaken, Amsoil's warrantee is longer than Triumphs..
(the Amsoil guys on this website can verify this)
I'm not going to get into a debate over which is better, Mobile 1 or Amsoil, I'm sure both are good, I just choose Amsoil, i'm not going to pay the dealer over $60. for oil that Mobile 1 stuck a "T" on their bottle and paid triumph to sell it for them.... when i can put Amsoil in (without the "T") for $30.