Please, please, please do not use the GM Orange stuff. It will sludge and gum up. I have seen that stuff make a mess. You cannot mix orange with other coolants. Actually the best solution is the yellow stuff, it will mix fine with all types (green included).
See:
http://www.google.com/search?hl=en&q...=Google+Search
Specifically:
http://www.consumeraffairs.com/news0...m_dexcool.html