Coolant Gerex G05 for 3.8 engine?


I am about to change the coolant in my '63 OTS (3.8). I was not able to find the green coolant I used to get for my car but was recommended Gerex G05 concentrate instead, which is more of an orange color. I was able to read some, non conclusive info on this orange coolant in the archive, but what is the latest on using this modern coolant on our cars?


I always heard that it’s excellent for an iron block / alloy head.

I would do some on-line coolant research…should find answers of what is compatible with what metals…I’d do it myself now…but busy…so maybe I will later as I am curious as well…but have a look. I really don’t know what to think of or do with…replies such as above…“I always heard that…”.
…With all due respect Jack…you heard from who…a friend with a tractor? a friend who had a Model T? Let’s try to post what we know…and can back up with really good resources…These posts are used by many…and live on. Sometimes." In my humble opinion" (IMHO) is valid…when a topic is subjective…but as Sgt Joe Friday…on Dragnet…said…we just want the facts m’aam…just the facts. Nick

Thank you all for your immediate response - much appreciated. The guide provided by Paul is good for explaining the principal differences of these various coolants but does not answer my question in regard of suitability for the Jaguar 3.8 engine. Also: If the coolant in question is suitable, can I just refill the engine with it after draining the green stuff or do I need to do a thorough coolant flush so to avoid a chemical reaction between the old and the new coolant?

When you say you previously used green do you mean Zerex original green? I can still get that at NAPA and is what I am currently using. I figure it is what we used in its day. It is now rated at 5 years. I have also used Prestone fits all 5 year.

Experience: Many years with Saabs.

In my opinion there are lots of modern coolants out there that will do the job.
The problem today is compatibility between the formulations. A simple flush is not enough, too much residue left behind. I have played it safe for many years and just used the cheapest Walmart brand green formulation. Even though many makes say “can be mixed with any color” I don’t believe it. You can not mix OAT and HOAT even if they are both orange in color. In trying to get the correct coolant for my new cars I follow the spec. in manual. Then they keep changing the specs, in one original was orange now replaced by purple. Not easy anymore. I believe Mike Frank was a believer of G05.

Regardless of what brand you use, the most important thing is to flush and renew regularly. Maybe every other year. Because of the iron / alloy interface it’s more important than it was with old 'murrican iron blocks and heads.

Tom: Yes, the Green Stuff is what I was looking for. Perhaps I am going to check out other local stores for that before starting an experiment. I was just wondering if anybody here had some experience with the modern stuff in the Jag engine.


A mechanic that I have had a lot of trust in has for years used Prestone coolant that says compatible with all. Never had any call backs. The article is also a bit off mark when it says most coolant comes premixed. Concentrate is VERY easy to find and sold right alongside the premix

Ok as a conclusion I guess I keep looking for the green coolant. Thanks for all the input.


No need to overthink this. There’s nothing exotic or special about the XK straight six and any quality coolant will work fine.
I just refilled my 3.8 E-type (and my 328 GTB) with Peak antifreeze…never had any problems.



Is peak the one that is propylene glycol based? I used a PG coolant some years ago. Unfortunately had a sudden and pretty bad leak and because you can’t smell it like EG coolant I didn’t find out until the engine overheated😡

Maybe you can’t! I can smell a micro-leak of that stuff… drives my bus mechanic nuts!

It is also -similar- to what is used to deice aircraft, and has the same distinct odor.

It has absolutely no odor to me (BC, before Covid) unlike EG that smells a bit like maple syrup. Interesting how you can pick up an odor. I refused to use PG coolant ever again after that incident.


Peak coolant is ethylene glycol.


I went with No-Rosion, happy so far. Used the flush prior to coolant. Can be used with anti-freeze, but I just used RO water because I don’t need antifreeze.

Maybe this will help. They market all 3 types, so they don’t have a significant incentive to push one chemistry over another.

1 Like

Hmm… Ill hafta pick up some of that “maple syrup,” and see what it smells like to me.

Years ago, back in the 80s, NatGeo did the first large scale research about the sense of smell: the issue had a scratch-n-sniff section, for two noses.

One test splotch, to my then-GF, smelled like peanut butter: to me, it smelled exactly like cat urine…:face_vomiting:

1 Like