This is something that is very hard to prove. It's the same as claimed improvements in suspension configuration. Very difficult to measure and very subjective.

My long standing 'Oils man' from my days in the MG clubs had a very different view on lubrication to most people, but that was his job. All I ever used in my Rover V8's was Valvoline 20/50 and with regular changes I had no problem with black sludge and the inside of the engine stayed clean.

I had a long e mail exchange with him about oils for my V6, and the impact on oil pressure, and his advice was basically to yes, use decent oil, but not to worry too much about the detail. I use fully synthetic, but always buy from Halfords - usually Mobil.

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4