Engine oil additives come in two distinct 'flavours'.
Flavour No 1. (Hummingbird Pheremone Flavour)
This you add to your existing oil to enhance performance, reduce fuel consumption, increase HP, re-rejuvenate worn parts and make you and your vehicle 'nearer to God than thee' (choose your own Nirvana(s) here) 
For the conspiracy theorists they add the components removed by the Castro loving, war mongering political/military/industrial government types who want your engine to wear out prematurely so as to maximise their filthy capitalistic profit (wave flag here whilst holding hands and singing Kumbuyah) whilst totally ignoring the fact that it's the same oil companies selling the additives that you've supposedly been deprived of in the first place.
It helps to have some psuedo scientific names to throw about here along with a picture of a dolphin or two, smiling children (of different ethnecities) and an inference that our hummingbird pheremones are better than our competitors non organic hummingbird pheremones and that pollution will be lessened to leave you with a nice fuzzy feeling about how politically correct/socially responsible you are being 
All of the above totally ignores the fact that the vehicle manufacturer and the oil companies probably have half a clue when it comes to specifying the correct oil for a vehicle and assuming the OP's D2a has run OK for the past 16 years on the manufacturer specified oil why mess with it ? In other words WOFTAM.
Flavour No.2. (Chocolate)
Aah ............. Chocolate, a good sensible choice for most ice cream devotees 
Supplementary engine oil additives can be useful when trying to 'compensate' for the effects of wear and tear/ageing in an engine. A good example of this is the seal softeners sold by Wynns, Penrite etc and particularly Risoline that can 'restore' old hardened oil seals to effectively reverse a leaking rear main or timing cover seal. Very cost and time effective (when they work). They won't fix a damaged seal but can be 'magic' when used on an old hardened seal. Another example of a good effective additive is the example given by Sieraferry earlier, a tappet noise quietener. There are others out there that I am sure 'work as advertised'.
The difference here is that these 'chocolate flavoured' additives are used to mitigate or hide symptoms of engine ageing, usually in older, worn engines that may not be an economic proposition to repair normally or where a temporary solution is required. IMO when used correctly these additives are not a WOFTAM 
Deano
66 SIIA SWB .......73 SIII LWB diesel wgn
86 RR 'classic'......99 Range Rover P38a
94 Defender 110..95 Defender 130 Ute
96 D1 300TDi.......99 D2 TD5 (current)
04 D2a Td5..........02 Disco 2 V8
Bookmarks