Right then Keith. I have had a trawl about and been on a few sites including http://www.castrol.co.uk which gives some good info.

Yes it is a direct replacement / upgrade to RS 10W60.

Below is something I lifted from GT-R.com which explains a bit more about it. But there is plenty of other info out there if you want it. One good thing is, I know what to use in the M3 now

"Replaces Castrol Formula RS 10W-60

Race proven for highly stressed and competition engines, as well as modified older vehicles

Product Features
Delivers race proven power and performance
Provides strength in reserve at extremes of performance
Stays in grade
Product Benefits
The best protection for older modified vehicles
Allows you to tune engine to the maximum
Provides protection when running fast, hot or hard

EDGE Sport 10W-60

SAE 10W-60 Race proven for highly stressed and competition engines

Applications

Castrol EDGE Sport 10W-60 is a hard wearing product for use in a wide range of sports performance engines. This product has been tested in race engines to deliver protection in hot and hard running scenarios.

Castrol EDGE Sport 10W-60 is suitable for tuned or high performance vehicles.

Features / Benefits

Delivers race proven power and performance for highly tuned, competition engines.

Provides strength in reserve.

Protects even when fast, hot or hard running.

Synthetic technology."