Formula, whether low iron or added iron, provides an exogenous source of
iron to the baby that interferes with baby's use of iron in breastmilk.
The problem is SUPPLENTATION with formula, not how many mgs of iron formula
contains.
And personally other than the major supplementation issue, I don't think
what you use to supplement matters all that much.(gasp).
As Jack says "fix the breastfeeding so supp isn't needed".  Or pump.
Sincerely, Pat in SNJ