If the lead flashing is good quality and to the relevant BS then it should be 99.8% pure lead or better.

As others have pointed out small amounts (1-3%) of other metals, typically tin or antimony, in the mix will change things quite a bit - and some of the changes can occur over time or even vary over time. The older wheel weights also had Arsenic in them which allowed heat treatment of lead alloys so you can quickly get into issues if you do not know the real source of your casting materials.

Keeping the mix at a constant temperature helps using something like a PID (but not sure how much with pure lead, I only use 'alloys').

Regardless of the mix I would flux, stirring with a dry hardwood stick seems to work for me, do whatever you're happy with, as there's always crap in there of one form or another.

Do you have access to the hardness gauge of any kind, found these indispensable in the long run?

Brgds Terry