You didn't mention the year/make/model of your engine… which could affect your choice of additive (if any).
I add Power Service Diesel Kleen to every tank for my personal truck and also in my company's fleet of (six) diesel vehicles. Sliver in summer. White in winter.
I used the stuff available at Wal-Mart sold in white square plastic bottles. Forgot the name but the bottles are white with black and red print. With the garbage they are peddling for diesel now, the stuff may help fuel mileage and driveability. I sure wouldn't pay for an additive unless it did something concrete.