Old wheel weights contain several percent antimony. New wheel weights vary: the stick on type are mostly lead, and soft enough to cast RBs. The clip-on types can be zinc, iron, or lead. Iron won't melt, zinc is much lighter and will contaminate your good lead. You can check each with a pair of side cutters to see if they mark at all, cut hard, or are easily marked (lead).
I've cast with cable sheathing and sheet lead. Depending upon the source, it might contain other than lead and tin (solder joints). IIRC, antimony was added to enhance extruding, and copper may be present. If you observe a pile of oatmeal looking mess on the top of your lead, skim it off! If more of the stuff floats to the top as you add to your pot, skim it off! It only takes a small percentage of antimony and copper to make the metal almost unuseable for roundballs.