I worked at a company once which had to recall a
whole lot of boards because the flux was growing a
white "mold" of some sort. Probably shoulda cared
more about cleaning, but MIL systems specify down
in their copious fine print "non-nutritive to fungus",
for reasons. Might check that one with whichever
vendors you're seriously considering. But what to
clean is likely still plain copper or gold wash and
I imagine most fluxes would do (you might make it
easier by some board prep to remove casual oxidation
or return boards too oxidized for "solderability" (a
spec you will also sometimes see on packaging
leads).
I like Kester 44 (60/40 lead bearing) myself.
Flux is for getting through native oxides, primarily.
So what's oxidized and which flux effective, is a
proper question and you hope solder mfrs have
handy answers for what they sell.