Although you can mix summer and winter washer fluid, it's not recommended. Summer washer fluid is typically thinner than winter washer fluid, so mixing the two could result in a subpar cleaning solution. In addition, summer and winter washer fluids often have different formulations designed to work best in their respective seasons. For example, summer washer fluid may contain more detergent to help remove bugs and road grime, while winter washer fluid may contain more antifreeze to prevent freezing.