Coolant Types

[Forrest]

Apologies, this is a coolant version of my recent battery thread.

I have read quite a lot recently about how modern coolant formulations are unsuitable for older cars. As I understand it there are three types of coolant available now.

IAT - Inorganic Additive Technology. This is the traditional (green/blue) ethylene glycol and silicate coolant which has to be changed every year or two because it becomes corrosive with age. Safe for use in older engines.

OAT - Organic Acid Technology. This is the modern (red/orange), phosphate-free, long-life coolant which can corrode components in the cooling systems of older cars from day one.

HOAT - Hybrid OAT. This seems to contain a mixture of organic corrosion inhibitors, ethylene glycol and in some cases silicates. Seemingly safe to use in older engines and lasts five years or more.

Now, as far as I can make out, Volvo blue/green coolant and Comma G48 which are both widely advocated for older Volvos and VWs respectively are both HOAT rather than IAT.

It seems a bit odd to me that the OAT can cause problems on its own but not when mixed in a hybrid solution. It seems especially odd that the classic community mantra seems to be to use IAT coolant when what a lot of people are actually buying appears to be HOAT. In some cases HOAT formulations appear to claim backwards compatibility with earlier (e.g. G11) standards.

Can someone please explain?