Just thinking aloud....
I wonder if higher mileage/usage cars have so much diesel going through their fuel system that it's lubricating and cleaning itself as it goes.
Conversely, low usage cars have comparatively little "lubricating and cleaning" so they benefit from the diesel with more additives?
Similarly we talk about doing an "Italian tune up" and there's loads of anecdotal evidence that this helps to "flush through" the injectors.
I have nothing concrete to base this on, it's just a thought to see what others can add to our knowledge and understanding.
