some molds will work better with pure - but I've used tire weight for years down to 1/32nd. You get weights with Z, ZA and lately I've seen some with ZF on them toss them. When I do any jig with crane swivels I try to use a 2-1 mix ( 2 lbs tire weight to 1 part pure). I pour 4-5 times with out hooks to heat my molds up and keep the setting on 7 on my Lee pot.
With lead prices any time you can get good tire weights for free you take them. Also helps to flux your lead makes it pour better. I had never done this until I read the thread here on TU. I don't smoke my molds or use release spray and have never had any problems.