Greg, I just add about 1/2 cup of liquid car polish to my fine walnut media. I let it run a half hour or so to fully mix it before tumbling brass. But you do not need to get anything expensive. The cheapest stuff like NuFinish or Turtle Wax will be fine.
Once that is done, give it about 2-3 hours in the tumbler. When it comes out, have your welding mask handy!!