Ford themselves recommend this:
http://www.ford.co.uk/OwnerServices/...LongtecA30W-40
Castrol EDGE Professional A3 0W-40, specifically tested by the Focus RS engine developers, meets the requirements of this uniquely tuned engine. This gives Ford Focus RS drivers the optimum level of protection and performance.
Castrol EDGE Professional A3 0W-40 is the sole recommendation for all new Ford Focus RS cars.