i have been doing a bit of research on this recently in relation to diesel.
the UK government has said that all UK suppliers of diesel must supply up to 7%bio diesel by 2015. bio diesel can be hated by modern diesel engines. including my 159 TI 2.4. until last year, the car companies said that they wouldn't warrant diesel with over 5% bio mix. as it does damage to injectors, pump and can cause algae build up in the fuel tank.
My father in law runs a BMW 325d convertable. he has always run on morrissons. he has recently changed to shell. he now has a quieter, more economical and quicker BMW. so, he rang BMW and asked why? they told him that although shell and BP run 'up to' 7% bio, the bio they use is different to the cheaper supermarket fuels. The supermarket buy in, pre refined fuel from the continent, mixed with rapeseed oil. this contains water, and less lubrication. BMW admitted that they have recently found 40k mileage cars that have been run mainly on supermarket fuel, with algae build up in the tanks. this is due to the water.
Shell, and the main forecourt brands, all refine in the UK, get tax breaks for doing so, and therefore can add FAME bio to the diesel, and not be too expensive. FAME(google it, wikipedia is good) has almost zero water, and has a high lubrication rating.
I recently bought my 2009 TI from a dealer with 41k on it. It is currently being dismantled by the dealer as it is coked up to death. they are convinced it has been caused by poor fuel.
There really is a massvie difference between the fuels sold by forecourts, and supermarkets. last year, asda and morrissons were both selling biodiesel B30. This is 30% bio. it was quickly removed from sale due to the failing of modern diesels(DPF equipped) both companies were very close to many court cases against everyday people that had filled up, and screwed up their cars.
I wouls srongly advise googling the above points, it makes interesting reading.
'07 GT Blackline JTD, but a 159TI 2.4 on its way very shortly!