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Making sure you're using the right oil for your 4WD is vital.


We’ve been asked many times what’s the best oil type to run in new and older model 4WDs and it’s not easy to come up with a straightforward answer. First, here’s some background into all the confusing acronyms that surround the oil business.


1926 Ford Shell Oil Tanker – Alexander Migi


Back in the olden days, specifying oil for engines was pretty easy. Then like everything else, it became complicated. From the early 1980s engine stress levels increased and oil formulae developed to meet those stresses. In recent years, oil formulation also had to help achieve lower emissions, as well as not interfering with exhaust after-treatment kit.

Oil bottles that once said merely ‘oil’, were decorated information labels and those labels meant a lot to chemists, but not to many other people.

The important factor to remember is that selecting the right oil for your needs requires input from two different classification systems: one covers viscosity and the other covers performance levels. In an ideal world, we’d have just one criterion, not two, but that’s just the way things developed during the 20th Century.





We’re all reasonably familiar with the Society of Automotive Engineers’ (SAE) classification for oil thickness, or viscosity. Oil container labelling typically says ‘SAE 15W-40’, or similar. However, we’re willing to bet that many 4WD people – including technicians – don’t really know what that means.

Years ago, engine oil had a viscosity rating that indicated its ‘thickness’ in an engine operating at an engine coolant temperature that was always assumed be 1000C.

So, a ’40-weight oil’ (SAE40) was ideal for a specific engine at normal operating temperature, but thick and sluggish at low ambient temperatures, hampering lubrication until the engine warmed up.

Then, the chemists got busy and we had ‘multigrade oil’ that was modified with viscosity chemicals that allowed it to be more free-flowing at cold ambient temperatures. Multigrade oils are universal for diesels these days.



As the accompanying SAE viscosity chart shows, commonly used 15W-40 multigrade diesel engine oil is rated to behave like an SAE15 oil, with good flowing characteristics at ambient temperatures down to around -20oC, yet with optimal engine lubrication at engine operating temperature of 1000C. (The ‘W’ means ‘winter’.)

Complicating the viscosity situation is an old-fashioned myth: ‘thicker oil is better than thin’. Many technicians believe that oil rated ‘15W’ is too thin to offer lubrication protection at normal engine operating temperature, whereas what they should be concerned about is: ‘thick oil causes cold-engine damage’. 

At normal engine operating temperature, both SAE40 and SAE15W-40 have the same viscosity.

For any engine, the manufacturer produces a chart of recommended engine oil viscosity grades for the temperature conditions likely to be encountered.

If that was the only consideration, using the right multigrade viscosity oil in your engine would be easy, but it’s not that simple. Enter performance classifications.



Before the arrival of the modern highly-turbocharged, electronically-controlled-injection diesel in the late-1980s, engine oil performance criteria were specified by American Petroleum Institute (API) Service Classifications: API CC (naturally-aspirated and two-stroke) and API CD (turbocharged).

Then came API CE, CF and CG classifications for oil that could combat piston deposits, valve train wear, oxidation resistance and soot accumulation.  

Emissions laws dictated more oil quality improvements and we scored API CH-4, CI-4 and CJ-4 classifications that were compatible with Exhaust Gas Recirculation (EGR), PDF and catalyst systems in engines introduced from the early 2000s.

API ‘C series’ classifications are ‘backward compatible’, meaning that oils complying with the latest standard can be used safely in prior engines. 

That includes the 2017-introduced CK-4 oils that are designed to address greenhouse gas emissions and to meet the needs of 2020s’ exhaust treatment systems. 

However, older oils aren’t ‘forward-compatible’ , so using CH-4 and CI-4 oils in post-2017 engines may ‘plug’ the diesel particulate filter, causing excessive regeneration episodes; ‘poison’ the catalytic material in the after-treatment system and jeopardise engine performance and life.


At the same time that CK-4 was introduced, there was a new API FA-4 classification, specifically covering post-2017 engines. FA-4 was deliberately given an ‘FA’ code, to distinguish it from ‘C series’ oils, because FA-4 oils are not backward-compatible, unless approved by the engine maker. An example of manufacturer-approved backward-compatibility is Daimler’s approval of FA-4 oils in some of its pre-2017 USA-market engines.


To help distinguish the two new oils the API has introduced new ‘donut’ logos that some oil makers put on their containers. The FA-4 donut has a red stripe behind the ‘FA-4’ type.

So far, we’ve considered only API performance classifications, but there are others.

In Europe, the European Automobile Manufacturers’ Association (ACEA) engine oil specifications were introduced in the early 1990s and they tend to have slightly different viscosity and exhaust after-treatment performance and longer oil drain requirements.

The Japanese Automotive Standards Organisation (JASO) has also developed specifications and some tests are are specific to Japanese diesel engines and Japanese emissions equipment.

Some iesel engine manufacturers have also developed oil performance specifications. Some specify a maximum sulphated ash/sulphur/phosphorus (SAPS) level in the engine oil.


Synthetic oil


Synthetic lubricants are manufactured using chemically modified petroleum components or synthesised from other raw materials, but the base material is still crude oil that is distilled and modified.

Synthetic lubricants were first synthesised by German scientists, because of crude oil shortages for military machinery during World War II. It also didn’t hurt that synthetic oils remained fluid in sub-zero temperatures of the Eastern front, while straight-petroleum-based lubricants of that time turned solid.

It was also found post-War that synthetic lubricants worked well at the very high temperatures experienced by jet aircraft engines. By the mid-1970s, synthetic motor oils were formulated for automotive applications and the same SAE viscosity rating was applied to synthetic oils.

Synthetic oils are derived from different grades of refined crude oil bases, but ‘synthesising’  creates molecules that retain good viscosity at higher temperatures, with branched molecular structures that allow flow at lower temperatures. 

Because of their improved viscosity index, synthetic oils don’t need so many viscosity index improvers and don’t degrade as quickly as traditional motor oils. That means potentially longer periods between oil drains.

However, synthetic oil is more expensive than straight-mineral-oil lubricants. Traditional mineral oil has developed in parallel with synthetic oil and it’s interesting to note that even the latest high-demand performance specifications can be met by additive-fortified, CK-4 and FA-4 mineral oils.


Why the new FA-4 oils



Designed as upgrades to API CJ-4 oils, both API CK-4 and API FA-4 oils reduce carbon dioxide and greenhouse gas emissions, while improving oxidation resistance, shear stability and aeration control. 

API CK-4 oils maintain traditional high temperature/high shear (HTHS) viscosity and provide performance benefits exceeding today’s API CJ-4 engine oils.

API FA-4 oils are of lower HTHS viscosity and flow more quickly through the engine. That lower flow resistance comes with fuel efficiency claims of up to two percent.

The downside is the need for workshops to have different grades of oil on hand and that has slowed the uptake of FA-4 oils. We’ll keep monitoring the progress of FA-4 oils in Australia.


Choosing the right oil



Remember that any oil is defined by its viscosity and performance level.  

Until the release of FA-4 oil for late-model engines, all previous performance classifications have been ‘backward-compatible’,so using a late-specification in an older engine hasn’t been a problem. Now, care needs to be exercised.

The engine manufacturer’s recommendations are the safest guide to correct oil selection, but in the case of aged engines that original recommendation may not be available. An older diesel engine may need only a Diesel Engine Oil meeting API CI-4, or even API CH-4 or CG-4. 

It’s best to confer with the engine maker’s technical department to see which of the modern oils the maker recommends.

Our thanks to the SAE, API, Wikipedia and Steve Streater, technical advisor for fluids and lubricants at TransDiesel Ltd, in the preparation of this article.




























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