Many fuel compamies (i.e. esso diesel synergy) use additives in their fuel to clean the engine and improve performance.
My question is whether these additives will harm the machine after 5-6 years of use ?
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It all depends on where you buy your gasoline. Not all gasoline brands are the same when it comes to quality.
According to AAA, they report that
"among brands tested, non-TOP TIER gasolines caused 19 times more engine deposits than TOP TIER brands after just 4,000 miles of simulated driving.”
In other words, at the cheap off-brand stations, you are less likely to get as good of gasoline.
Their report also states
having enhanced, engine-cleaning detergent additives apparently does make a huge difference, as they prevent carbon deposits from negating fuel economy, lower emissions, and positively impact vehicle performance in newer vehicles
which answers your question about engine damage.
Fuel has had additives for years and years, lead being one that was added to reduce knocking then removed for health reasons...
The damage that may or may not be caused by additives will be very well hidden by the servicing regime : non-existent, mediocre, good and the driving style : shopping-trolley, short commute, sales rep (long highway / motorway use).
To sum up the additives won't cause damage, unless one is buying the ones in supermarkets (gogo juice, injector cleaner etc) and adding too much (not reading or following the instructions...).
Personally my car is 13 years old and is fine... I stick to one quality brand of fuel and avoid the "cheap" supermarkets - well once in 3 years as it was too close to empty...