Sludgy Anti-Freeze

11K miles ago, I put in a near-zero mileage 3.1 into my '95 Cutlass Supreme that had been salvaged and sat for 12 years. Also installed new hoses & radiator but left the old heater core. While changing out the thermostat the other day, my mechanic noticed that my anti-freeze was sludgy. He recommended getting the coolant system flushed and to put new anti-freeze in. He also recommended not using Dex-Cool, opting instead for the green stuff. Why is my anti-freeze sludgy and is this advice sound?