Phil wrote ...

Well... it's now complicated

One can get very complicated about coolant and I was once told to simplify it into green coolant and pink coolant, the latter being an extended life antifreeze. However, there are intrinsic differences in the constituents of various coolants and their compatibilities with one another.

Types of Anti-Freeze

There are three primary types of anti-freeze for your car’s radiator cooling system:

1. Ethylene Glycol/Polyethylene Glycol. Tried and true, this type of anti-freeze is still standard after almost 70 years. Modern ethylene glycol anti-freezes have several additives to protect, clean, and lubricate your cooling system.

2. Non-Toxic Radiator Coolant. This anti-freeze does not contain ethylene glycol or polyethylene glycol (both acutely toxic). These engine coolants are, however, still toxic but only in larger quantities and over longer periods.

3. Extended-Life Anti-Freeze. In most cases, this is simply an ethylene glycol anti-freeze with one or more carboxylate additives to prevent corrosion of the cooling system. These engine coolants should make your car radiator and cooling system last longer.

Common engine coolant additives include: nitrates (corrosion inhibitors, buffers), silicates (corrosion inhibitors, especially for aluminum radiators), carboxylates (buffers, corrosion inhibitors), and borates (buffers).

Never mix different types of engine coolant. You can damage or further reduce your car radiator’s efficiency by mixing the different types. Before you change types, thoroughly drain and flush your cooling system.

The different types - made even more complicated:

OAT coolant (orange or pink) contains no silicates and no phosphates. It's a blend of two or more organic acids, a specific class of inhibitors with slow-acting, long-life properties. Texaco's Havoline Dex-Cool (also sold under the Goodwrench label by GM) was the first example. Prestone and Peak also have introduced OAT coolants that are chemically compatible with Dex-Cool.

Conventional Japanese coolant (green or red) contains no silicates, but has a heavy dose of phosphates and other inhibitors, including a modest amount of one or two organic acids.

Conventional European coolant (blue or yellow) contains a low dose of silicates and no phosphates, but does include other inhibitors, including one organic acid.

Hybrid European coolant (blue or green) is similar to conventional European, but with a much greater dose of organic acids. It's a balanced formula designed to have the silicates provide the primary protection for the aluminum, then allow the organic acids to provide long-term protection.