I did a little experiment. (Don't tell my wife, but the kitchen was my lab).
I use Prestone 5/150 in my bus, and Zerex 5/100 (not phosphate free) in my
Dodge cargo van. I was curious if my tester would be calibrated the same for
both brands. Under controlled conditions, I mixed up a 50% batch (fresh) of
each solution, and tested it with a floating dial type tester. To my
surprise, the needle read slightly different - not a staggering amount, but
consistently different, the orange stuff showing a higher freezing point.
Both solutions are ethylene glycol based, so there must be differences in the
other additives.
I just thought someone else might find this interesting.

Regards,

Brian Doss
'88 Weekender, Queequeg