This article comes thanks to Mark Tiddy in South Australia, who has been researching coolants for both modern and classic Lotus cars.

Mark Tiddy

As I am about to replace my Radiator, introduce a new remote thermostat and replace a couple of cooling system hoses, I started to explore what coolant I should be using and in more detail verify the “claims” around some coolants being linked to deterioration of silicone based seals etc.

My study involved a long discussion with a very knowledgeable technical consultant at Nulon. The person I spoke to had a very detailed and in depth knowledge of coolants from a chemical, practical and mechanical perspective, so I have absolutely no doubt that what he explained is indeed correct and I have verified it with other research.

Firstly I have learnt that you cannot go by colour alone. Different manufacturers have different chemical compositions so it is difficult to put “all reds” or “all greens” in the same classification, so I will try to keep this general. Secondly, you cannot depend entirely on “vehicle manufacturer recommendations” “if” you have made any changes to your cooling system.

In general terms, red coolants are (mostly) classified as “OAT” coolants (Organic Acid Technology) and they seem somewhat “newer” than older green coolants.

Many manufacturers now specify these OAT-based coolants. Many (but not all) OAT-based coolants contain a chemical referred to as 2-EHA (2-ethylhexanoic acid). This chemical has been the subject of great controversy regarding the fact that it is a plasticiser and has been proven to soften silicone materials within the Cooling system. There are many cases in the USA where extensive litigation has occurred due to engine failures that were attributed to this and in fact it has cost some manufacturers hundreds of millions of dollars in engine failure claims.

Another important attribute regarding OAT-based coolants is that chemically, it does not react well to some metals that may be in some cooling systems such as brass, copper & solder.

In more modern times, manufacturers have ceased using these metals (changing to entirely aluminium) and can therefore confidently specify this as the approved coolant because they know the construct of the entire cooling system, including heater cores, pipework etc. So if a cooling system is entirely unmodified and the manufacturer specifies to use an OAT coolant, then you can safely continue to use that coolant.

Now over to (again in general terms), the green coolant. These coolants are more generally ethylene glycol coolants and more recently some brands include a blend of organic and inorganic additives (sometimes referred to as hybrids).

These coolants are more tolerant of (and in fact designed to work with) cooling systems that have such materials as copper, brass etc. According to Nulon, another benefit to their green hybrid coolant is that it has slightly better properties regarding the sealing of small capillary leaks that may start to form in older pipework, seals etc so is the better choice for older cars. Tests have shown that a very fine blemish that might exist in a seal, head gasket etc has a greater tendency to weep with red OAT coolant than will with green non-OAT.

So in deciding which coolant you should be using, the answer firstly lies in whether the cooling system has been modified or refreshed in any way and you have introduced different materials into the system. If the system is entirely unmodified then it is safe to use what the manufacturer specifies. If, however, you have introduced silicone pipes into the system, then you should not be using a coolant that contains 2-EHA, otherwise you risk deterioration of those components.

The good news is that the fallback option is that the newer green hybrid coolants (such as Nulon’s Green Long Life Coolant) is compatible with ALL systems.

Confusing?? Yes, in many ways. There’s even blue, yellow, orange etc, so my advice?

1. If your car is factory specified as requiring a particular coolant (whether it be OAT or otherwise) AND your cooling system is unmodified/does not contain any silicone pipes, then use what the manufacturer specifies. (Note that some Automotive manufacturers coolants are different chemical constructs than others so to be absolutely certain, you should in fact stick with the brand of coolant the manufacturer specifies. A good example of this is that both Toyota and Honda’s own brand coolants apparently are red “OAT” but definitely do not contain 2-EHA. There is no guarantee that a generic or other brand red OAT coolant will or won’t contain 2-EHA, so one should always check)..

2. If your car is older and/or contains known components that have brass, copper etc in them (ie thermostats, heater cores, radiators etc), then use a green hybrid coolant.

3. If your car was factory specified to use OAT (probably red) AND you have replaced some pipework with silicone pipes (ie. new radiator hoses, the introduction of a PRRT Remote Thermostat etc) then EITHER find a red OAT coolant that definitely does not contain 2-EHA OR change over to a green hybrid coolant (like Nulon Green Long Life Coolant)

4. If your car was factory specified to use green ethylene glycol-based coolant, then continue to use a green hybrid coolant. Under no circumstances change over to an OAT coolant.

Link to Nulon’s Coolants.

Note: If you buy the 100% concentrate coolant, it needs to be mixed 50/50 with demineralised water. Supercheap Auto have a special that started today where you get a 6L bottle of 100% concentrate Green Long Life Coolant for $39 and it includes a free bottle of radiator flush.

What am I doing? Well, seeing as my new Elise Parts-branded radiator hoses are silicone and I’m installing a PRRT (which includes silicone hoses), I have just purchased the Nulon Green Long Life Coolant and a bottle of demineralised water.

On another related matter, given the link between 2-EHA to the deterioration of silicone, one ponders whether this is (in part) the cause of the older style Rover head gasket failure where there is a fine bead of Silicone between the oil and water galleries. Clearly “if” this were partially responsible then one would expect that it’s only likely to have occured in cars where people were running OAT coolants with 2-EHA in them. Having seen some examples on the internet where this silicone seems to have gone soft on gaskets, I’m not taking the chance. Fortunately my Rover-engined car has been using green coolant so I don’t think that’s been a risk in my car.