By preference I use Shell V Power in my Plus 4, or other non supermarket high octane fuels if it's not available.
In theory the higher octane fuels will allow the engine ECU to run greater ignition advance which will give greater efficiency and power, particularly at higher revs. Fuels like Shell V Power also contain additives and detergents that help keep the fuel injectors clean and minimise carbon deposits on the valves and in the combustion chambers (thus optimal conditions).
Running the engine on 95 RON fuels, especially in the lower rev ranges and under load, the ECU will be backing the ignition timing off against the knock sensors (i.e. running a more retarded ignition curve) which will reduce both power and torque, and efficiency (hence reduced mpg).
The gap between the theory and practice however, can in real terms be minimal (the odd BHP or ft/lb of torque, and maybe 1 or 2 mpg) it tends to be dependant on how sophisticated the engine management system is, what you want from your car, and of course your driving style.
So, as ever in the Morgan world... each to their own
P.S. Originally a reply to Charlottendal, but Richard and Ruud got there before me... Slow typing!
