It's time for me to flush the coolant. I am wondering what to put in. Dex-cool again? Prestone Extended Life? Ol' greeny? Strawberry Quik? I have noticed a bit of brownish sludge on the walls of my coolant overflow reservior, which makes me hesitate putting dex-cool in again. I am leaning towards Prestone EL, since I heard it is like dex-cool but isn't dex-cool. Any suggestions?