Poodleshooter. Plain old parafin will work as a flux, as will any old candles that you have laying around the house. Be aware though, that they will smoke like hell. To eliminate most of the smoke, light the fumes with a match, or as I do, I "flick" my BIC lighter. The same one I use to smoke my molds.
You can keep your mold hot by casting a little faster. Sticking it in a flame can warp your mold making it useless.
The commercial fluxes work very well. I use Marvelux, available from Brownell's.
If the metal you are using is very dirty, you might want to flux, and skim the dross from the melt, flux again and wait about 5 to 10 minutes to let what little junk is left in the melt to come to the surface. Them skim that off. I use mostly wheel weights for my casting purposes, and I get large amounts for free. (My son works for a car dealership.)I have to flux that stuff 3 or 4 times, and even then it still has junk in it. Makes good bullets though.
In your case, with "pure" lead, fluxing will clean the metal and make it cast just a little bit better.
Good luck with your casting efforts.