There aren't any "Mechanics in a can." Most of them are BS. However, running a good injector cleaner once a year is not a bad idea. With the detergents most gasolines are using now, engine deposits are at a minimum. I ONLY buy two products, one for oil system and one for fuel. Marvel Mystery Oil for oil. Run a can through every 12-15 thousand miles. Add this can along with the normal amount of oil when you change it next time.
Second, for fuel, buy a couple of the pourable cans of Berryman's B12 Chemtool. Add the appropriate amount to a fresh tank of gas. For that tank, your fuel enconomy might drop a little, but after it's run through it will be better. B12 has been used for years cleaning carb's and as an additive.
The only other additive is if you are going to store fuel for extended periods of time (more than a month or two), and that is STA-BIL.
These three, very inexpensive additives should be in everybody's garage. Hope this helps.