Anything that contains carbon will work.
For use in a large pot for what we call "smelting" (or for casting in a bottom pour furnace) I prefer sawdust and lots of it. I throw it in from the start to let it get reduced to ash and another handful or three as I add more metal to the pot. I especially like sawdust that contains resin and the more the better. It keeps things extremely clean and the smell is natural and more like a camp fire. Sawdust works really great with a bottom pour pot when casting and I leave a layer on while casting. After fluxing, just throw another good handful back on top of the pot and let it smolder back down to ash as you cast and it will be ready for the next fluxing.
For ladle or dipper casting where the flux has to be removed completely, the sawdust works, but petroleum products is faster. I normally just use old candle remnants or about anything that will not stink too badly. Watch for flare ups.