CJ-4 vs. CI-4: Which Diesel Oil is Better?

It is common to hear people discuss the relative benefits and drawbacks of the newer CJ-4 diesel oils vs. the older CI-4 diesel engine oils. So, it seems worthwhile for us to clarify a few things about the differences between these oils.

Many of our customers are often seeking our AME 15w40 Synthetic Diesel Oil, either because they are afraid of moving to a "lighter" viscosity 5w30 or 5w40 oils or because they've heard "bad things" about the new CJ-4 oils.

Key Additives Reduced in CJ-4 Oils

For the record, our AME 15w40 oil dos not meet the latest CJ-4 specs because to meet CJ-4 specifications lubricant manufacturers had to significantly reduce key additives involved in acid fighting (lower starting TBN).

Extended Drains Compromised?

As a result, it is assumed that the extended drain capabilities of CJ-4 oils have been compromised as compared to CI-4 oils with higher TBN values in order to make the oils more compatible with the new EGR systems on 2007 and newer diesel vehicles.

These newer EGR systems are putting HEAVY soot load on the oil, which has necessitated an increase in detergents and dispersants in the new CJ-4 oils. In addition, low-tier CI-4 oils (petroleum based and low grade synthetics) have had to improve their basestock quality in order to met CJ-4 specs. however, if you're comparing premium synthetic oils, the basestocks are likely pretty similar to their CI-4 counterparts since these basestocks were already plenty capable of meeting CJ-4 requirements.

So, in most respects CJ-4 oils ARE an improvement over CI-4 oils. The only POTENTIAL drawback to using a CJ-4 oil is a shorter oil drain. however, if you don't take your vehicles to the limit of your oil anyway, then you won't likely notice the difference. For instance, the 15w40 is guaranteed (and I've seen it easily last) for 25,000 miles or one year.

End Users May Not Notice

However, MANY customers change it long before then because they don't fully trust the oil for those intervals and don't want to pay for oil analysis to verify it. Hence, even though the CI-4 15w40 oil MIGHT offer a longer drain than our CJ-4 5w40, it's likely that most end users will not notice because they never take their oil to the limit anyway.

In addition, there seems to be evidence that, although the starting TBN value of the CJ-4 oils is lower than CI-4 oils, TBN "retention" seems better with the CJ-4 oils. Thus, it is possible that, even with the lower initial TBN, the CJ-4 oils may still offer similar drain interval capability. Only time and more testing will show this for sure.

So, the end result is, If you don't intend on trying to take your oil to the full 25K/1 yr drain, it's likely that you won't notice the drain interval difference between the two oils. On the flip side, wear protection and detergency of the CJ-4 oil is going to be better. Thus, for many users, the CJ-4 oil MAY actually be the better choice, even if you own a pre-2007 diesel which doesn't require it.

Which Oil for Which Engines?

AMSOIL 5w40 DEO Synthetic Diesel Oil is our CJ-4 oil that CAN be used in all diesel engines, both older and newer - it's completely compatible with all of them. Of course, it's primarily recommended for newer 2007+ model year diesels, but is compatible with the older models as well.

I would not, personally, recommend a CI-4 oil, such as our AME 15w40, in a 2007 or newer diesel. Technically, it does not have the detergency properties of the newer CJ-4 oils to handle the excessive soot load that these new EGR systems are going to place on it. If you're going to stick to short drains, you'd probably get away with it, but I'm not sure it's the best recommendation.

But You Want a "Heavier" Oil?

As for the issue of viscosity, many are wanting the 15w40, which, is one reason they may be shying away from our CJ-4 5w40 oil. Keep in mind that, at engine operating temperature, both oils are virtually the same viscosity. They both qualify as an SAE 40 oil at engine operating temp (around 200 degrees F). The 5w40 actually has a kinematic viscosity of 15.3 cSt at 212 degrees F, whereas the 15w40 has a kinematic viscosity of only 14.2 cSt at 212 degrees F.

Thus, at engine operating temp, technically, the 5w40 is actually a "heavier" weight oil than the 15w40. And, of course, at cold temps, the 5w40 oil will perform much better than the 15w40 will, as a result of it's 5w classification.

So, in the end, if you're looking for a heavier weight oil, the 5w40 is "HEAVIER" once the engine is up to operating temp. And, because AMSOIL uses such a high quality basestock, it's 5w40 oil will hold up VERY well in the presence of shearing forces, which means it will maintain its viscosity extremely well.

Hopefully, this will clear up any confusion about which diesel oil will be best for your application.