NuFinish liquid car polish (not paste).
I add two capfuls to pre-treat new fine grit walnut media (24 grit) and run the tumbler until all the clumps are gone (I even stir the media with a stick to speed up the process).
I add another capful every 2-4 batches and it works well to clean my range brass (about 20-30 minutes) and polish (about 45 min - 1 hour+). If I want even more shine, I will polish using corn cob media and NuFinish.
Benefits are the residual polish on the brass acts like case lube to help with resizing pistol cases when using carbide sizing dies and also helps prevent brass from tarnishing and maintain the shine.