Hi everyone, I have a 2004 Silverado 1500 4.3 V6 5 speed, 40,000 miles and the coolant is al bit dark and i would like to flush it out. What coolant shou;d i use, the prestone premix dexcool or the Zerex dexcool formula ? i'm a tech and have seen some gm cooling systems sludged up,but i think...