I have no doubt that a good dose of various products on the market added to the fuel give real gains in terms of improved performance, sometimes economy, noise reduction etc. Some are quite good at shifting carbon based deposits from valve stems, injectors etc.
The real issue is how long does the effect last for and how often do you need to re-treat and what is the cost element.
I myself have always used and promoted the addition of small amounts of 2T oil to both Diesel and Petrol tanks as they have in my experience given similar benefits - noise reduction, small improvement in fuel economy etc.
Indeed I had a Petrol W124 series 260 E Mercedes saloon a few years ago that would never start instantly especially in the winter. It always needed a bit of cranking 3-5 seconds before it started.She had done 115k when I bought her so I was not expecting miracles but if I added 500 ml of cheap 2T oi to the tank every month or so she would start instantly hot or cold. When I sold her she had 184k on the clock t was a pre-cat model.
Where I work part time at a MB dealership we have in the past have had a number of products promoted via Inchcape to give new life to older engines. These have included the use of engine flush products (that I personally would never use) and fuel additives.
As a footnote if the additive is particularly good you might need to do a fuel filter change as some of the gunk might be shifted downstream so as to speak. I know those who run on veg oil have issues with its cleansing properties in the fuel lines.
:thumbs
