Mixing a green coolant with DEX-COOL reduces the batch s change interval to 2 years or 30,000 miles, but will otherwise cause no damage to the engine. In order to change back to DEX-COOL however, the cooling system must first be thoroughly drained and flushed. While there have been intake gasket failures on CK Series, V8 powered vehicles for various reasons, DEX-COOL has never been found as a cause. Its inhibitor package is strong enough that if the batch still provides proper freeze protection, it is probably still providing proper corrosion protection as well. DEX-COOL can handle the minerals in hard water better than silicated conventional chemistry coolants.
Drinkable water is suitable for top off.
In ST Blazer applications where the radiator cap is mounted at an angle to the ground, the vehicle is more susceptible to radiator cap contamination and its related problems. The Stant 10230 is a wise choice for these vehicles. A check with local cooling system shops revealed this cap. It's from a mid-90 s Blazer with about 50K miles. Below is the neck.