Coolant isn't just dyed water and glycol — it's a carefully balanced chemical package where the corrosion inhibitors matter more than the base fluid. Get the wrong chemistry in your engine and you'll eat water pump seals, dissolve solder joints, and pit aluminum heads. Here's what those cryptic acronyms actually mean.

IAT (Inorganic Additive Technology) — the classic green stuff. Uses silicates and phosphates as inhibitors that coat metal surfaces with a protective film. Effective but the inhibitors deplete fast (~2 years or 30,000 miles) because they're consumed as they work. Silicates also drop out of solution as gel, clogging radiators. Found in pre-1996 GM, Ford, Chrysler.

OAT (Organic Acid Technology) — Dex-Cool orange, VW G12, most modern European. Uses carboxylic acids (sebacate, 2-EHA) that only react at corrosion sites, so inhibitors last 5 years/150,000 miles. Downside: 2-EHA is a plasticizer and attacks certain gasket materials. It also can't protect against cavitation as aggressively without silicate help.

HOAT (Hybrid OAT) — the compromise. OAT carboxylates PLUS a small dose of silicate for fast aluminum protection. Ford's yellow "Gold," Mercedes G05, most Chrysler pink. Best of both worlds for aluminum-block engines.

The mixing disaster: Combine OAT with IAT and the silicates react with the carboxylates to form a brown mayonnaise-like sludge that plugs heater cores. This is the actual mechanism behind "the coolants aren't compatible" — it's not marketing, it's precipitation chemistry.

Real-world example: The infamous GM 3.4L V6 Dex-Cool intake manifold gasket failures weren't primarily the coolant's fault — they were caused by low coolant levels exposing the plastic gasket carrier to air, which oxidized the 2-EHA and formed acidic byproducts that then ate the gasket. Keep the system full and pressurized, and Dex-Cool lasts fine.

Rule of thumb for glycol concentration: A 50/50 mix drops freeze point to about -34°F and raises boil point to 265°F (at 15 psi cap pressure). Going richer than 70/30 actually reduces heat transfer — pure ethylene glycol has roughly half the specific heat of water. The math: every 10% increase in glycol above 50% cuts cooling capacity ~5% while giving diminishing freeze protection.

Testing hydrometer freeze point isn't enough — the glycol survives but the inhibitors deplete silently. Use test strips that measure reserve alkalinity and nitrite/molybdate levels for diesels.