I believe the majority of the UK guys with older engines (not the LS series or Ford equivalent) use 20w50 either Valvoline VR1, or Millers Classic 20w50.
I use 20w50 Valvoline in my 383 SBC (have also used Millers Classic)
These both have more ZDDP in than the others which i understand the older engines require (don't quote me on that, i'm not an engine builder, it's just what i've read/been told
).
Check out the UK cobra club forum for a fair number of threads of this very subject
rgds
Gary