I'd take it to AutoLusso near Dunstable, in Bedfordshire.
Their phone number is 01525 221921.
I took my car there from York for the fitting of a Q2 diff and top wishbone arm, their mechanic is brilliant.
AutoLusso use Valvoline oil, which has a good reputation as well.
If your in N.London you could go to Veloces or Alfarama, they would have selenia, i would recommend Selenia Racing 10w/60 or Shell Racing 10w/60, i had 5w/40 in my v6 but made the engine a bit ticky when it got really hot, moved to 10w/60 and the problem dissapeared.