Between diesel and Optima diesel I did, there was 50 miles more in the tank for the optima, this is not an observation but from Lossie to Lincoln this year in an LDV (gawd help us all) I got further on 2 tanks than I did the previous year.
Significant reduction in smoke and a smoother idle with Shell Optimax diesel in mine. A lasting benefit too, stays good for a few tanks of normal fuel. No economy benefit though.
I did get good economy on a tank of Total Excellium but that could have been the long trip I did on it.
Both worth it for, say, one tank in five if available at a reasonable price I reckon.