10w30 preferred always, and if it's below freezing all the time you can use 5w30. That's what the book says.
Slick 50: I tried it many years ago on a very older car and it seemed to help with mpg, but I didn't see any other benefits and quit using it, much as Rat did, after all the bad press about it.
I've always just changed my oil at regular intervals, never had a problem.
One product you might benefit from is
this one. I can personally recommend it; I've used it on two older vehicles that I bought and it works as advertised, both in engine and trans.