Anti-freeze - Almost all use ethylene glycol (EG) or propylene glycol (PG) (newer) for the antifreeze base.
Anti-corrosion -
Conventional - Green/Yellow antifreeze contains silicates, phosphates and / or borates as corrosion inhibitors to keep the solution alkaline. This is called Inorganic Additive Technology (IAT)
European car makers like VW specify coolant additives lacking in phosphates and including borates and low silicates (such as VW's G11 (blue)) because their water is harder and it reacts with phosphates to create aluminum phosphate or calcium and magnesium deposits.
See more on problems with phosphates at autorepair.about.com
The Japanese disagree and specify mixtures high in phosphates and low in borates and silicates because they fear lack of maintenance will cause borate corrosion.
Tap water in North America also contains calcium, but isn't as hard as European tap water so phosphates are considered okay to use here.
Organic Acid Technology (OAT) or Extended Life Antifreeze Coolant (XLC or ELC) antifreeze contains neutralized organic acids (organic acid salts) that protect engine parts from corrosion. These include Havoline® XLC, GM's Dex-Cool® (also manufactured by Texaco) and VW's G12.
Honda has an extended-life OAT coolant that is dyed dark green and does not contain 2-EHA.
OAT includes such ingredients as sebacate (Toyota & Honda), 2-ethylhexanoic acid (2-EHA) and other organic acids, but no silicates or phosphates (except in the case of Toyota's pink extended-life coolant, which adds a dose of phosphate to its extended-life OAT-based antifreeze).
Organic inhibitors do not deplete as quickly as the inorganic inhibitors used in conventional coolants.
OAT is recommended in engines with aluminum components (heads or radiators) because Borates can be bad for them.
OAT coolants also have extended lives, up to 150,000 mi.
OAT corrosion inhibitors provide excellent long-term protection for aluminum and cast iron, but may not be the best choice for older cooling systems that have copper/brass radiators and heater cores, especially the lead solder used in them. It depends on the formula.
Hybrid OAT (HOAT) coolants, such as "Zerex G-05®" Use organic acids, but not 2-EHA (2-ethylhexanoic acid) (different organic acids are used). Hybrid OAT coolants add some silicate to provide quick-acting protection for aluminum surfaces. Silicate also helps repair surface erosion caused by cavitation in the water pump. Hybrid OAT coolants are currently used by many European vehicle manufacturers (BMW, Mercedes & Volvo) as well as Ford and Chrysler.
Toyota adds some phosphate to their hybrid.
The new universal coolants use unique OAT-based corrosion packages with proprietary organic acids (such as carboxylate) to provide broad spectrum protection.
