I would guess it is a sponge. I used a sponge.
You should use something that will not "break" into pieces. Towel maybe. Paper towel better not.
Think of all the fluff that would get stuck in the nozzle.
A sponge worked for me BUT I have read (on the internet) that you SHOULD NOT USE A FILAMENT OILER because the oil will leave particles in the nozzle that will eventually clog the nozzle.
I only use this without oil to get rid of dust particles that are clinging to the filament. I would recommend that.