When BP released 'ultimate' diesel I ran my van for over 1500 miles on it to see if it gave better mpg.
Result: No better mpg. No smoother running.
Previously Mrs_G ran our then car (Mondeo 2.0 si) on Shell Optimax. This consistantly gave 20+ miles extra range before the fuel light came on. This more than compensated for the extra cost.
I have used Bp Ultimate constantly for over 18 months and 20000 miles from new in my 1.9 multijet cargo.
Whenever i switched to supermarket (tesco) derv ( emergencies) even shell extra it was noticible after a while how smooth and more oomph not mpg the ultimate gave.
But unfortunately due to the escalating cost Ultimate nw 111p a gallon near us now I have switched to standard bp diesel as have found it suits my van best with a shot of new millers super 4 cetane booster.
Wow ! now been running for 500miles and it raises the cetane level by 4 points and promises all sorts and it really does! far more responsive and higher free reving engine better than ultimate so now it costs me about another 2 p a litre instead of 6p with ultimate and get a better drive.
So my advice use a good quality diesel with a shot of millers and get the best of both worlds for less.