Re: Additives ( a discussion)
IN my neck of the woods Stanadyne are the favored products.
Additives will help 'boosting the Cetane rating' of the fuel
... if your engine is 'old' it was designed for diesel fuel that had much higher cetane (think octane in gasoline) rating than present ... higher cetane, slower fuel burn and less 'knocking' under load.
... Lubricity -- older engines depended on the sulphur (now almost totally removed from #2 diesel) to lubricate the high pressure injection pump. Have you priced a new or rebuilt high pressure injection pump lately?
... emulsifier to break up water into 'teeny' or macromolecular size ... prevents injector tip erosion.
... biocide - you already have bought the T shirt on that one. No discussion needed other than keep the MINIMUM amount of fuel on board + some reserve (dont always fill the tank ... it has a relatively short 'shelf life' when exposed to atmosphere), buy FRESH fuel from a high volume truck stop.
for an older engine .... definitely use additives unless you are lucky enough get your fuel from a non-USA source.