Prepared to be shot down in flames - as "my mate in the pub told me"

One of my mates who works for Saab tells me that Tesco get a lesser grade of petrol that the likes of Shell, BP, etc petrol stations. Something to do with "seasonal fuel" - apparently in summer the fuel has either more or less octances (or something) - and Tesco get what is left from last "season". He will never use Tesco - but always goes to Shell, etc while for obvious reasons I use Tesco all the time and he tells me while it is cheaper - you get less miles to the gallon - so it is a false economy (but he does not take BA miles into my equation).

He told me this while drunk in the pub - so might be a load of nonsense.