Marine coolant types: OAT vs conventional (IAT)

Never mix coolant types. Two chemistries dominate: (1) IAT (Inorganic Acid Technology, conventional green): traditional silicate/phosphate inhibitors, 2-year service life, universal for older engines. Volvo Penta, older Yanmar, older Cummins. (2) OAT (Organic Acid Technology, red/orange): carboxylate inhibitors, 5-year or 4000-hour life, required for newer engines. Cat ELC (red), Fleetguard ES Compleat OAT (used by Onan and modern Cummins), Volvo Coolant VCS (yellow, OAT variant). Mixing IAT and OAT creates a gel that clogs the heat exchanger and destroys water pump seals. If unsure what's in the system, flush completely with distilled water until runoff is clean, then refill with the OEM-specified type. Concentration: 50/50 with distilled water for most climates, up to 60/40 in extreme cold, never above 70 percent glycol (heat transfer drops). Test annually with a refractometer or coolant test strips (glycol concentration AND pH, pH below 7 means the inhibitors are exhausted, coolant is corrosive regardless of glycol strength). Never use automotive coolant with silicate, silicate abrades marine water pump impellers and drops out as sediment.