A long time ago, when I was in the Air Force, somewhere between the Wright brothers and yesterday, I was assigned to an outfit which flew Super Constellations.
We had a guy in our outfit that used to use avgas for his car and cigarette lighter, because he was too damn cheap to buy the right stuff.
Well, one day he lit a cigarette, and the lighter went "poof" in his face. A few weeks later, when he got out of the hospital he tried to drive home, and his car wouldn't run. Seems he burned the hell out of the valves and the cylinders!
Moral of the story?
If you want tumbling media, buy TUMBLING media! Not cat litter, swine food, or any of that other stuff.
I have two Lyman 2200 tumblers. The first had Lyman treated walnut, the second has Lyman treated corncob. Brass gets 1 hour in the walnut then 1 hour in the corncob.
The Lyman treated media can be purchased for around $1 per pound, depending on how good a buyer you are. And 12 pounds of each has lasted me for years!
Do I add anything to it? Hell no...
When it takes longer than an hour for the stuff to do it's job, it gets tossed and a new batch is dumped into the tumbler.