Best additive for any diesel is two stroke oil. Your looking for a low ash miniral based oil.
Best stuff I could find off the shelf was Castrol Actevo 2T for mopeds.
I always added 250mls to a full tank of diesel. The two stroke forms and emulsion with the diesel, helps keep all the pumps and injectors lubricated, ups the cetane rating for a cleaner burn, cleans everything out and keeps them clean! The engine will also run smoother, quieter, smoke less, performance should be up a little and should improve fuel economy.

Used to run it on my old clattery 1.9TD 406 which is an indirectly injected engine with a mechanical fuel pump. With a little two stroke it was quiter than my Uncles 406 2.0 HDi which is a commonrail directly injected engine!

If you don't like the thought of two stroke the only other additive I really liked was Millers Ecomax which you can buy from most motofactors or halfords. Usually around £10 for a 500ml bottle. You add it at 1:1000 or 1:500 for an extra 'boost'.
Works out at 1ml per litre or 2ml per litre!

Oh and make sure you put the additive in BEFORE any fuel goes in the tank. Otherwise it wont make it to the tank and will just sit on the sides of the filler neck!