Two days ago, I asked the guys at Opie what they recommended for mine once the new engine's been run in and and they came back with this:

I would use a 5w-40 synthetic once it's been run in.

http://www.opieoils.co.uk/c-656-5w-40.aspx

Out of those, the best ones are the Fuchs/Silkolene Pro S, Millers CFS/CFS NT, Motul Sport/300V, Red Line and Gulf Competition. The Motul 8100 X-Cess, Millers XF Longlife, Fuchs GT1 XTL/Supersyn, Gulf Formula G/ULE, Shell Helix and Mobil Super 3000 are good, cheaper alternatives.

But mine will be only running about 300bhp, so that may not hold true for a tweaked motor