Had issues with supermarket standard fuel from sainsburys a while back, stopped using it and then I got 6p off a litre this year voucher. Thought why not. Used it in my car and lawnmower. Lawnmower hated it, car wasn't the same. Had a word with the lawnmower service guy and he said not to use it unless going through a tank fairly quickly. There are barely any fuel stabilisers in supermarket fuel which is why it tends to go "off" within 2 weeks. Never had an issue with branded fuel like BP, Shell, Esso in the tank even after 4-5 weeks
Son's Merc doesn't like supermarket petrol, runs rough with a higher idle and very prominent lag over 3k rpm. Most people won't notice this as they hardly go over 3k rpm anyway. Petrol engines also seem to be more resistant to crap fuel.