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I do mine every 2 years. Pop the hoses, let it run with a fresh water hose to clean it out real well. Then use about 10 gallons of distilled water for a final flush, drain it down 2 gallons, put the orange stuff in and the GM veggie tabs, then top off with distilled water. Make sure and keep the OPTICRAP dry and to burp the system!
I would replace what was installed from the factory. My 91 had the green stuff so I replaced it with that (and added a bottle of water wetter). GM did'nt start using dexcool until 95 or 96 so your 90 should have had green coolant in it. I flush and replace the coolant in all my cars every two years (or at a cooling system failure). Did the vette in 06-01 and will do it again in the spring.