Trim Microsol 585XT is run in all of my sumps, as per Nathan the Machinist's recommend. Been using it for a couple of years now (same batch for the most part) in my big horizontal saw, my mill's sump, and surface grinder.
It's easy to maintain, low foam, no funk, and pretty much non-toxic. Unlike the soluble oil based coolants, it doesn't require constant maintenance or aerating to keep it from growing alien death bacteria. Some of my machines don't get used weekly, let alone daily, and all I have to do is occasionally add roughly mixed make-up solution, or even just water to bring the concentration down.
I don't even bother with refractometer mostly.
Only issue I've seen is that it's pretty rough on most of the traditional paints used on machinery. Although many other coolants are also. I'm guessing these are oil based paints?
Anyway, highly recommend this stuff.. I've used a handful of traditional coolants and synthetics, none of which I like as much.