I still don't understand why people go to the trouble of sorting though a pail of WW to remove zinc and steel - if you enjoy it and have the time, that's fine, but it's not necessary, and in fact you're bound to miss some anyways. I did that for my first pail, but for the several tons of raw WW I've done since, I only removed the stickon WW's, and those that I noticed that were obviously zinc or steel - as I ladled them in. All you have to do is pay attention - as soon as the melt goes from "mush" to "liquid", you ladle out the waste that will be compromised of steel clips, zinc and steel wheelweights and of course the odd lug nut, screw, etc.
My newest WW is about three years old and most is five or older. I have yet to encounter less than a 70% conversion ratio (raw to ingot), but the two pails I have yet to process are the newest and the ratio might be lower.
I always sort out the zinc afterwards from the waste and make up zinc ingots, just for fun and to make the odd zinc bullet in the past. It takes a lot of time and a lot of heat to melt them - heat so high in fact that any lead that might be in with it had long since melted and was glowing red. It's also a bugger to cast them too, as the heat level is way above anything that would work for lead, and the bullets are quite light for their size, and brittle.