Green coolant should be changed every two to three years. It should be changed BEFORE it looks like it needs to be changed.
Personally, I use OAT (Dexcool) in my Contour and HOAT (hybrid OAT) in the Escape (GO5). The Escape came with HOAT and I don't see a need to make a change.
Most important of all, I change them both annually, during the aniversary month.
