2003 Buick Regal Owner's Manual - Page 245

Adding Washer Fluid Notice: When using concentrated washer fluid, follow the manufacturer's instructions for adding water. Don't mix water with ready-to-use washer fluid. Water can cause the solution to freeze and damage your washer fluid tank and other parts of the washer system.Also, water doesn't clean as well as washer fluid. Fill your washer fluid tank only three-quarters full when it's very cold. This allows for expansion if freezing occurs, which could damage the tank if is completely full. it Don't use engine coolant (antifreeze) in your windshield washer. It can damage your washer system and paint. 5-40