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Do any products or additives really improve engine performance?

this is NOT a specific question regarding a model of car, but my question is of all the liquid oil and gas additives that do everything from clean the engine to make the gas mileage better, is there a product or two that actually functions in making the engine run better or smoother or are the products based on smoke and mirrors?

I have used a few that provide some benefits. Techron fuel injector cleaner. Just came in from using Seafoam to clean the intake. I use only synthetic oils and lubes so I don’t add stuff to the oil. I have a buddy that swears by Marvel Mystery Oil for his Mazda rotary.

That said, virtually no serious problems can be fixed by magic mystery snake oil additives. There are no miracle cures for worn out parts.

My opinion on this: a vehicle is designed to run and maintain at it’s best using manufacturer recommended fluids If you want to add additives it’s usually to try and correct a problem. My stance is to fix the problem so you don’t need the additive. If you decide not to fix the problem because the correct fix isn’t worth saving the vehicle, then using additives as a last ditch effort is acceptable.