I own a 2006 Chevy Impala with a 3.5-liter V6 engine. The odometer is at 25,000 miles and the coolant has never been changed. The other day when I checked the overflow tank the coolant was low so I added a gallon of green anti freeze to the tank. I told my mechanic this and he said to have the system flushed immediately or trouble will crop up. Is this true?