For the last three or so years I have dieseled up my Subaru Forester at our local Jet station. Recently, I had to do it at a Waitrose where I filled up with Shell. My average mpg on Jet had been around 43, but by the time I had used half the tank of Shell it was nearly 46mpg. I went back to Jet and it fell again to 43, so, after a couple of tanks, I tried Shell again with a similar result. I think the car runs better on Shell too and the price is similar. Is this a surprising discovery?