This is a good test to read. It was undertaken by John Thorne (who I know from the old BM3W forum) and he ran his fuel test over a period of 12 months. It proved the value of Tesco's 99 octance fuel using an M3CSL as the donor vehicle..

Original test:

http://www.thorneymotorsport.co.uk/tuning/Fuel_Test_Results.shtml

Update, published more recently:

http://www.thorneymotorsport.co.uk/tuning/Fuel_Test_Results_Update.shtml