StanadyneI add a little Stanadyne to my fuel at the recommendation of my diesel mechanic. Theoretically the fuel suppliers add additives to restore what lubricity was lost in the shift to ULSD a couple of years ago. I don't always added it but I do most of the time. I seem to get a little (0.5 - 1.0) boost in mpg but nothing scientific and probably nothing to off set the cost. Since my engine is from the LSD and not the ULSD age, I figure it can't hurt. If I had a new engine designed for ULSD I'd probably skip it.
Buy it at Rural King in 1/2 gal bottles for $22 and it will treat 250 gallons so it costs me around 11 cents per gal.