I've always used untreated walnut shells, and an overnight tumble will result in some shiny brass come daylight, kinda.
But over the last 3-4 years, because of some good advice from some reloading pros, I've started squirting a couple thimble-fulls of Turtle Wax Premium Polishing Compound (Autozone) in my walnut media prior to tumbling. In 2-3 hours, the brass is as-new and glistening. Reportably, the Turtle Wax leaves a wax coating that keeps the brass looking new much longer too. It's even brighter than brand new brass, if that's possible.
"For every complex problem there is an answer that is clear, simple.....and wrong." H. L. Mencken