Green antifreeze or coolant as some call it.

From Motor August 2004 "Coolant Confusion: Its Not Easy being Green, or Yellow, or Orange.."
by Paul Weissler
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and from p. 32
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A followup article from 2010 is also available as pdf download:
"Relearning the Alphabet: Making Sense of the Cooling System Scene." also by Paul Weissler