I'm gonna take a stab at something, and I am probably completely wrong, but I have been thinking about this.
What is Dexcool? (Yes I know its coolant) but what is it engineered to do?
Lemme take a stab at my own question.
Modern engines are very heavily engineered and produce wacky amounts of heat in order to operate and pull a vehicle. The Series II 3800 in my estimation is a modern mill and thusly produces more heat then say an L27. Therefore a new method of cooling these super hot engines was engineered.
The resultant product = Dexcool. NOW, is it possible that if a Series II engine is running old skool green antifreeze, this product is not doing the job that it was engineered to do. Now common knowledge states that heat will cause a liquid to evaporate or burn off. So, is it possible that the heat generated by your Series II 3800 is burning off the green coolant.
If all the gaskets and intakes and other mechanical riff-raff has been fixed and accounted for, then maybe I'm onto something...
Or maybe I'm rambling.
I look forward to being proven wrong, I really want to learn here.