Avoid anything with ZINC. It'll contaminate your lead and the pot you have used to melt it.
Good soft lead is often difficult to find, so many of us from time to time, have experimented with questionable alloys, including dental lead and wheel weights. I agree that wheel weights can make balls suitable for smoothbores ONLY! It isn't even very good for cartridge bullets.
Our dental lead here makes into hard balls, or at least, harder than pure lead does.
Recently, I acquired some lead sheeting that came from a renovated x-ray department. It is dead soft...the best lead I've ever used. I managed to get nearly 500 pounds of it, so now, for the first time in my life, I can make lead:tin alloys that my BPCR's love, as well as have dead soft lead for my longrifles and pistols.