I bought a 97 f150 with 190K. Coolant was brown - I know it was a universal AMAM (Prime brand) because that's what old invoice said. I did 3 drain/run/refills one afternoon with conventional green coolant. Even after the drain/run/refills, I would still see brown crud floating at top of plastic degas bottle - especially after highway driving with heater going. It has gotten better the more that I drive - I scoop out the brown crud each time. Am I actually slowly continuing to clean the radiator and heater core by doing this or is the brown stuff oil mixing with coolant?