The difference is in the additive package. All coolants (besides RV coolant) is ethylene-glycol based. The old "green" coolant we know and love is phosphate and silicate based. This coolant requires frequent service and does have corrosion issues with newer dissimilar metals (i.e. aluminum, iron, magnesium, etc.). So, the OEM's tried to reformulate their coolants. OAT is Organic Acid Technology coolant, and is what GM uses. HOAT is Hybrid OAT coolant, which is what Ford, Chrysler, and Honda use. The differences get very technical, but suffice to say NOBODY wants to use Dexcool in their car but GM. Honda threw a fit when GM insisted that Dexcool was to be used in the 3.5's in the Saturn Vues. Here are a couple of links to articles by Motor Magazine. They're very informative and will answer a lot of questions.
http://www.motor.com/article.asp?article_ID=816and also, a more recent one...
http://www.motor.com/article.asp?article_ID=938Mark
