1995 Buick Regal Owner's Manual - Page 192

How to Add Coolant to the Coolant Recovery Tank If you haven't found a problem yet, but the coolant level isn't at 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. 5-18