On Sat, 22 Aug 2015 01:29:30 +0100, John Rumm wrote:

An expert that I trust says that supermarket fuel isn't as good as
other fuel. He says it's all down to the additives.

It also depends on the quantity of ethanol added - particularly on older
engines.

You won't find any petrol that's less than 5% Ethanol, and - unless
clearly labelled E10 - it cannot be more.