engine oil additives are useless.
fuel additives useful, especially in countries with crap fuels. I use Liqui Moly "acceleration" additives or even normal ones. The acceleration focused are great if you are willing to spend more money.
I usually combined fuel additives with ECU remap...reduced ignition delay, better combustion....without additives I get over 50km less on 700km fuel tank...especially, in third world countries such as Czech Republic where I was born...cetanes make noticeable difference over winter and at partial load at low revs.
EU diesel suppose to have 52 cetanes but there is rarely over 48 i n Austria/Germany.
Fuels have less sulphur, zinc because emissions...paradoxically, those two element are used as wear protection in engine. I never had injection system problems so I dare to say that they don't lie if they say it lubricates injection pump and injectors.