Some fuel additives can be very good.
I was so lucky that I was able to try the difference, in a unscientific real world test(that still goes on).
The result so far, was rather surprising, at least to me, since I have never thought the difference would be this big.
We bought an old Nissan Sunny 2.0 Diesel 1993 model, with 199000 miles(320400km) on the odometer.
Technically, it is in perfect condition, but it was running like [censored].
By the way, Mola; this is NOT the same car where I have used the Rev-X in the transmission(but I will do, as soon as I can).
The owner had always used the best diesel and oil(group 4 and 5) available, and it does not seem to use any oil.
But, he has NEVER used any additives in the fuel.
I had a bottle of F2-21, and decided to try it, and not do anything else(not even change the oil), for some 5-6 tanks of diesel.
The difference is rather surpricing, and can be seen here:
https://bobistheoilguy.com/forums/threads/f2-21-fuel-additive-makes-a-difference-in-one-car.124999/