Premium vs Regular

Most expensive fuel vs Cheapest fuel.
I recently read then I asked people who have actual cars and now I’m in the middle of “what do I believe”
I am about to purchase a 2003 Acura TL 3.2, (so I’d say its a old model)

I’ve read that although premium gives a slight change in mpg if I use regular gas something in the car will become “retarded” per se?

Also If I do save mpg and a full tank from premium and regular is about 3-5$ different why not?

What do you guys think?