When the car gets old and get a lot of milage, the dieselpump wears. Spesially if you'r area has a lot of winter diesel. (They add more paraffin and other stuff in the winter diesel that dries out the seals and outer components) So with time the fuelpump don't get the same lubrication and the seals leak more, the the pump struggle to maintaine the pressure.
With a hint of two stroke oil (mineral or syntetic dosn't matter, but mineral is cheaper) in the fueltank, the two stroke oil lubricates the pumpe better and help the dieselpressure.
There is no scientific answer for the mix ration.
If it's been a while since i have used two stroke oil in the tank, i start with 2-3dl in the first tank and take less and less for each tank until i reach 0,5-1dl and stick with that.
This is roughly, but about a small cup at start and a half a cup for the rest.