The answer is a definite no. Ford specifically states in their manulas and maintenance guides that Motorcraft Premium Gold coolant is not to be mixed with any other type of coolant, including their own Premium Orange and Premium Extended Life Green coolants. Dex-Cool and other "Universal" orange and green coolants are not to be mixed either, as these do not contain the specified additives Ford requires for continued cooling, anti-corrosion, and seal lubricating capabilities.
Either green coolant or red ECL (extended life coolant), depending on year and make and model of engine.
Either universal green or red ELC (extended life coolant), depending on what engine it has.
The Cummins ISX uses red ELC (extended life coolant).
General Motors changed to Dexcool Extended Life coolant in 1996. It is the orange colored coolant. Green coolant will not hurt the system, it must be replaced about 5 times as frequently though. Beware that once you use green coolant in your cooling system, it is very dificult to remove completely if you want to use the extended life coolant in the future.
It is not rust, but broken down extended life coolant. When Extended life coolant goes beyond the replacement time, every five years, it can break down and turn into a rusty sludge like substance. I would bet if you haven't changed the coolant in the car, it has never been changed.
You should be using red extended life coolant (ELC).
extended life "red coolant' Tell the people at the parts store and they will get you the oem coolant (do not mix red and green)
You may use Volkswagon G12 coolant or Prestone Extended Life coolant. If cost is a factor use the latter.
The 1998 Ford Ranger Owner Guide shows Ford Extended Life Engine Coolant ( Dexcool )
Extended Life Antifreeze which you can get at any retail locations
Toyota Long Life coolant. Pricey. Any brand name ethylene-glycol based coolant, mixed 50/50 would be OK.