If you question whether the lead is clean just flux it with some candle wax. Shave some wax off a candle, a heaping tablespoon full is plenty, and drop it in your molten lead and give a stir. Wear gloves doing this and DO IT OUTDOORS. Any impurities will come right to the lead’s surface where you can use an old spoon or the ladle to remove them.
Blended lead will work fine for larger sinkers and jig that don’t have any essential small crannies that need filling. If you get incomplete castings on small jigs some sinker designs, start over with pure lead.