IAT (Inorganic Additive Technology, green) uses silicates and phosphates, lasts 2 years/30,000 miles, and suits older cast iron engines. OAT (Organic Acid Technology, orange/red/pink) uses organic acids, lasts up to 5 years/150,000 miles, and is designed for modern aluminum-heavy engines - but should be pH-tested every 30,000 miles, since aluminum's pH sensitivity means coolant within its mileage interval can still be chemically failed; change immediately if pH tests below 8.5. HOAT (Hybrid OAT, gold/yellow/turquoise) combines both technologies for a 5-year/100,000-mile life and works best in mixed-metal systems.
Aluminum reacts with both acidic and alkaline coolant, so its inhibitor package depletes faster than in cast iron, and aluminum engines generate stray electrical current in acidic coolant that accelerates pitting corrosion. Mixing OAT and HOAT coolant causes gelation that clogs the radiator, heater core, and coolant passages - always flush completely before switching types.