Thick radiator fluids = old, old, old style coolant WAY pats it's prime; two old types of incompatible coolant mixed and emulsified (doesn't happen if mixing incompatible modern coolants... progress eh?); oil contamination from a BHG; OR a coolant system LOADED with a HG leak stopping agent.
A bad water pump will usually move coolant... just not enough.
Yes, a BHG can cause overheating... it contaminates coolant allowing oil/water sludge to create an insulating coating on the coolant passage walls and typically allows coolant to exit the system to the point that flow is reduced beyond serviceability.
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