petrol (and diesel) have to meet a british standard spec at the pumps. As a result oil companies will make to that spec with very little giveaway. In reality the main difference is the 'performance' additive that each company adds (or in some case doesn't add). Every company makes their own claims but you will find the majority use similiar additives but put it in at different rates somewhere between 250 and 450ppm. Most of this is to keep the engine clean (ie detergent agent of some kind) Throw into this the reproduceability of the test method allowing for .59R and there is virtually nothing in it. As for running on Super Unleaded or 'normal unleaded it depends on how efficient your engine is as switching between the 2 grades some ECU's compensate for it quicker than others. Most oil companies supply each other and only the additive might be different.