Castrol LMA has been synthetic for a while now, not the vegetable oil-based fluid it was years ago. I think anyone using it at this point is doing so only for nostalgia purposes, presuming they can even FIND any nowadays. My area PepBoys auto parts stores stopped carrying it 2 years ago.
Valvoline synthetic should be similar enough to Castrol synthetic, I'd expect. Everyone who hasn't been able to get the Castrol for years seems to have replaced it with the Valvoline, judging from the results of a google search using "Castrol LMA vs Valvoline."
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).