Why do you need an additive? are you trying to prevent/cure bug, sludge or fix dirty fuel? or are you smoking a bit? or just want to prevent any problems?
The most common type is excessive blue due to carbon build up. The best known cure is to run the engine hot and hard for a period of say 20 minutes or more. This is the best bit of regular maintenance you can do to any diesel engine.
Next time you see a BMW driver biffing it, remember it tells us in the manual that we should drive fast at every opportunity after a period of town driving, to blow out deposits from low speed running.
If the smoke stinks acrid of hot, part burned diesel you may have an injector or two with a poor spray pattern. Easily diagnosed by pulling them and reconnecting in reverse (ie sticking out) and turning the engine over whilst observing from a distance of a couple of feet. You should get a nice puff of diesel mist. Any squirts (keep well clear because of this) or dribbles, get it serviced and reset - cheap as long as it's truck / agricultural specialist who is doing it.
If it is burning oil there are additives for oil to swell internal seals for a stay of execution. Valve stem oil seals can be a relatively easy job to if you need a de-coke (head off). If it's piston rings then leave it until starting is a problem.
Most additives are right load of old voodoo but you could ask your nearest independent (back-street) LandRover specialist if they recommend anything to smarten up the old ones to get them through an MOT emmissions test.
The best stuff to reduce wear is fresh oil. Some additives claiming to do this actually cause more problems than they cure. Do a Google on 'miracle oil additive problems' and you will witness plenty of 'wwwevedence' to this. Same will probably work searching 'miracle diesel additive problems'.
Also beware of the oil grade you use. There is usually little to be gained except sometimes bigger problems, by using a modern, higher grade oil than the engine was designed for.
Try 'Forte' fuel additive. I have run 'oil burners' cars/boats for 40++ years Never
had any real success with 'additives' until i tried this. Will NOT make a zot of
difference to an engine with mechanical wear--sloppy rings -worn valve guides
etc-but a carboned engine runs like a 'bought one' after a dose of this. Never
had a car knocked back on emmissions (some with over 300k on the clock)
after a dose and burn out. Cost of a go-- about £10 -- available-decent motor factors