Surprised to hear Dex-Cool not available in some
locations--Dex-Cool is a General Motors spec and is required
in Saturns.  Should at least be available at a GM dealer.
It is same as Prestone Extended Life (silver bottle).

There is no problem in mixing with other types except that
the extended life feature goes away.  Actual statement from
Texaco is : "Compatible with conventional antifreeze.
Dilution with conventional antifreeze will reduce extended
life benefits. Texaco recommends that this product not be
diluted by more than 10% with conventional coolants."  Full
specs at
http://www.texaco.com/shared/products/havololant.html