The only additive I use is Fuel Set in my boat and in jerry cans where it may be stored for a few months. This stuff is not snake oil and absorbs condensation moisture that occurs in boat fuel tanks over time, and prevents algae growth.
There are other products around like injector cleaner that do work but are not be a miracle cure for a neglected fuel systems. They are more a regular maintenance item IMO if you want to spend money on them.
With common rail diesels I agree with an additional water trap and have one with a see thru bowl on my Hilux, but I would be cautious of advice to add additional filters without some research. Most if not all CRD's recirculate fuel back to the tank as a means of keeping the fuel pump cool. Any restriction to the return flow can cause fuel pump problems that may not become evident in the short term, and can also interfere with the electronic fuel warning systems.