that looks like untreated corncob - which is exactly what you want for normal polishing. You can use walnut shells, but they are really only used for heavy polishing.
Flitz is the only product I would ever use in a tumbler. You can try other products but you would need to double the tumbling time and they will not look nearly as good.
I use a dentist's pick set to remove the media from screws and use a magnet to remove small parts from the tumber.
I will guarantee that your new tumber will be your favorite tool in the garage.
