If in doubt, I look at the Halfords flip chart. Go for the equivalent age RR V8. I'm guessing it will list 5w-30 or 5w-40. The Rover V8 is not designed for high spec fully synthetic. For years I ran my '74 3.5 on Valvoline 20w-50, then switched to Castrol Magnatec which from memory is a 5w-40.
