Do not use Dexcool. It has plasticisers in it that are hard on gaskets and rubber components. It has a bad reputation for a number of technical reasons.
Generally speaking you are best to avoid any coolant labelled "extended life", for various reasons. Some are like Dexcool and are known to have technical issues. Some use different formulations that are not intended to be mixed, making "topping-up" a problem if you don't have the right stuff handy.
It's safest to use conventional green coolant. It has silicates. You cannot mix it with "extended life" coolants, but otherwise its pretty much foolproof. It's widely available, making "topping-up" easy and the same formulation is sold under many different brands. You can use it in pretty much all engines, regardless of whether they have aluminum, iron or brass components. (You can't use "extended life" coolants with brass rads, as I recall)
The downside is you need to change it every 2 years or so. But that's it,
Some Prestone yellow anti-freezes are virtually identical to the green stuff. Some are labelled "extended life" so its tough to tell whats in your car right now. But chances are its due for a change. Just flush your system with fresh water to get rid of the old stuff and use distilled water 50/50 with the green silicate coolant and you will be fine.
Heres an excellent article on coolant choices:
http://www.mastertechmag.com/pdf/200...antifreeze.pdf