My view is that if you buy a decent oil then the additives that you need are all in there already, and if you buy a poor oil and you are worried about what additives to use then the best thing you can do is start using a decent oil.
HOWEVER, I am a lay person and there are others on this forum that will have better answers.
S211 E320Cdi Avantgarde Estate & Toyota Land Cruiser
Loads of threads on this but in general terms view is that if you use quality synthetic oils at best a waste of money and at worst potentially damaging.
May be a case for specialist additives in certain circumstances but otherwise save the dosh and change the oil more frequently
In general forget oil additives. There appear to be two main types, thickeners/stabalisers and anti wear types.
Unless the engine is worn you don't need the thickener type and the anti-wear type normally have groound teflon particles in them.
Ground teflon is definately bad fro the oil filter and bearing surfaces and will often cause more damage than good.