Here in the UK unleaded petrol is offered at standard and “premium” grades - the price difference being about 8% - 24%. As far as I can make out the claimed advantages of “premium” grade fuels are that the premium grade provides:- 98 octane instead of 95 octane rating; friction reducing additives; cleaning additives; more MPG. It seems that both standard and premium grade petrol contains a proportion of ethanol.
I have been trying out the premium grade fuel for a couple of months in my 1971 Daimler Sovereign 4.2 litre and am struggling to experience any benefit - particularly given the average 16% additional cost. With these older pre - computer controlled engine systems and relatively crude carburettor fuel systems I am doubtful that the additional fuel cost is worth it. If I want to clean my inlet valves and fuel system I can always use a bottle of fuel additive from time to time.
What do others think?