A mechanic told me that I need to have a fuel.

The stuff you buy in a bottle is a concentrated version of what's already in the gasoline you buy. It might be nice for occasional maintenance but it rarely solves an existing problem. The stuff the shops usually use is a high-pressure can of strong detergent. They disable the car's fuel pump, then run the engine on that bottle for about 20 minutes. Some people have good luck with those products but it's effective mostly on GM products. Their injectors are not flow-matched at the assembly line so with high mileage and a little varnish buildup, it is common for them to develop a misfire that the cleaners can partially overcome.