I take that your not talking of the veg oil that some diesel owners use to reduce the costs of running a car.
Most diesel additives sold as such claim to clean out the system but to use them constantly could be prohibitively expensive. However, if your Rangie doesn't do many miles, then a bottle of something to raise the cetane level and a good blast down an open road is not a bad idea before the MOT bloke examines what you're shoving out your back end.
Then there's the 'unofficial' additives. The biggest, most used of these is probably two stroke oil. The thinking is that, to try and make the diesel at the pump less polluting, the level of sulphur has been reduced but the sulphur acted as a lubricant, like lead did in petrol. BMW did some trials and came to the conclusion that a 1:300 mix of two stroke gave a noticable effect.
Petrol in the diesel in winter can prevent waxing, and I've no doubt that there are others.