The diesel additive I recommend is Type F Automatic transmission fluid.  It
cleans injectors (detergent) keeps internal seals in FI pump in good shape, is
really cheap, and readily available.  Usually adding a quart a month is
considered common practice in the diesel world.  My friend who was a diesel
mech for several years, said that some of the other injector cleaners eat up
the FI pump's gaskets.
That's my 2 cents.
Ken Wilford (certifiable VW mechanic and multiple VW diesel owner- last count
we have 5 on the road right now besides my Vanagons)
Van-Again
John 3:16