I used to use the Castrol RS 10/60 ( now called Castrol Edge 10/60 ) - same as used in many E39 M5s.
Now I use the fully synth Silkolene Pro S 10w/50.
Both have done a good job for me

The Silkolene is supposed to be better at high temps according to Oilman ( Opie Oils ). He sells both but there seem to be more special offers on Silkolene