If you replace you should really be sure you've erased all traces of Dex-cool - it doesn't play well with other coolant. I've got Dex in a 2001 w 115K. It is perfectly clear - no sludge. I occassionally check the reservoir and make sure it is filled to the full mark. Every 4 years drain and refill the radiator. Use distilled water to mix 50/50 and make sure no air is in the system. Dex-cool is not as evil as people think. Air in the system or low level will cause problems - possibly worse and more sensitive compared to ethylene glycol and water.