I added the Peak I had around which was standard green. That is what it had in it, well green at least. The radiator was actually 1-2 inches low as well only noticed cause he temp gauges was dancing wildly, not overheating mind you. Not that it matters since everything I've read on the subject denotes the cooling system is oversized for the application anyway.
The manual in the truck was really vague as well, indicating to use just ethylene glycol coolant.
All is well, no issues. Used about a gallon.