Vehicle engines require coolants to keep them from overheating.
In the past, there was only one kind of coolant (anti-freeze) and that was the green liquid. Today, there are two: The green coolant and the orange colored coolant, which is also known as Dexcool. “Traditional” coolants (often green or yellow) generally use silicates, while “new style” (generally orange or pink) coolants use organic acids.
The two coolants should never be mixed together as they do not react well. When mixed together they can form a thick, jelly-like substance that can completely stop all coolant flow which can lead to overheating. As the coolant stops flowing, other problems can occur as well such as happens with radiators, water jackets, and even heater cores. The water pump may overheat and stop working. In severe cases, head gaskets can blow, and heads may warp. The engine itself will encounter major damage.
One small note, apparently someone from the SAE (Society of Automotive Engineers) said you can safely mix up to 15% of the traditional coolant type into an organic acid type coolant without much of an effect on the corrosion inhibitors. I’d still avoid it if possible. If you want to top it off just once, it’s best to get a bottle of distilled water and pour that in.
Here’s the bottom line when it comes to cooling system maintenance. Whether you are using the orange or green coolant you should inspect the coolant level and the condition of the coolant at every oil change. In addition, completely flush and refill the system every two years or 25,000 miles, whichever comes first. These actions will avert the problems associated with coolant products.