GMC 1994 Yukon Owner's Manual page 218

How to Add Coolant to the Coolant Recovery Tank
If you haven't found a problem yet, but the coolant level isn't at or above
the COLD mark, add a 50/50 mixture of clean water (preferably distilled)
and a proper antifreeze at the coolant recovery tank. (See "Engine Coolant"
in the Index for more information about the proper coolant mix.)
NOTICE:
In cold weather, water can freeze and crack the engine, radiator,
heater core and other parts. Use the recommended coolant.
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