Personally, I wouldn't put anything in there that doesn't come out of a diesel pump! Modern diesel engines are SO finicky about fuel quality! There are plenty of aftermarket potions available though. I've heard good things about BG244 and Stanodyne in particular. Also, a surprising number of good reports regarding adding a bit of 2-stroke oil to each tankful. I'd be tempted to regard this as an old wives' tale, but I've heard quite a few people raving about it on several different fora - about 200ml in a typical 50-60 litre tankful and claiming significant MPG increases after a couple of tankfuls.
I think they use O rings on injectors these days so that they're more tolerant of misalignment (and thermal expansion) between the fuel rail and the heads. Some of that might be cost-saving too, but personally, I wouldn't fancy trying to remove a 10 year old injector if it had been screwed in!