[submariner409]

Prestone's DEX-COOL is perfectly OK. Don't get crazy with extended flushing or chemicals. Just drain as much as you can (2 - 3 quarts will remain in the block as there are no block drains) and refill with a proper mix of DEX-COOL and distilled water (supermarket). GM has pretty much discontinued the use of the Bar's tablets (or the powder, G12BP), so if you had no leaks before you started, there's no reason to use them now. IF you run clear water through the system and then drain it, mix your coolant at about 60/40 coolant to distilled water to account for the water still in the block and elsewhere.