Its just a fuel additive afaik.
The dealer's pushing it because they can charge you 5x what its worth so take his recommendation with a pinch of salt - its rather like the engine flush mine tried to sell me, when pushed as to whether I REALLY needed it on a properly maintained, mobil 1 lubricated diesel with 60,000 on the clock? ...'no, probably not'.
However, if it puts your mind at rest, pop into any car spares shop and you can get a bottle of additive around £10 a bottle. It will do no harm. Lots on the market, some you add a measured amount each fill up, others the full botttle in one shot. Fill up, add the appropriate amount. Job done.
I have a bottle of Millers diesel sport that I add to mine (1 measure per 20L) every 2-3 fill ups, just to keep things nice and shiny. They do a petrol version I believe.
If its running a bit off and there's nothing showing up on the daignostics I'd be looking at leads as a more likely cause to be honest. Dealers seem happy to flog you expensive unnecessary treatments but mine repeatedly failed to spot weak ignition leads on my previous car. At 90k if they haven't been replaced I'd be seriously considering it.
I used to take my old diesel injectors apart. clean them in solvents and wipe clean.
Never done petrol, but have read the sides of many bottles in halfords of injector cleaner.
Majority are kerosene base, some are just diesel, many are a mix including surfactants.
I use my own additive in petrol now and again.
No more than a cupful per tank, but 60% acetone, Toluene and a splash of methanol.