Selecting the right oil for a hard-working diesel is more than picking a familiar grade off the shelf. High loads, long distances and Australia’s climate put serious stress on engines, so your choice should be guided by specification, viscosity behaviour and the conditions you face. For clarity, this guide focuses on the essentials that matter when comparing truck engine oils.
Start with the job the oil must do
Under heavy load, bearings and cam lobes rely on a protective film that resists being squeezed out. That film strength at operating temperature is described by high-temperature high-shear, or HTHS, viscosity. Modern FA-4 oils are blended to an HTHS range of 2.9 to 3.2 mPa·s to reduce friction and aid fuel economy, while CK-4 products sit higher for broader compatibility. Checking the data sheet for HTHS gives you a clearer picture than the label alone.
Know your categories: API CK-4 and FA-4
The API service category tells you how the oil was built and tested. CK-4 is widely approved for on- and off-highway diesels and is backward compatible with many older engines. FA-4 targets newer on-highway hardware designed for lower HTHS and fuel efficiency. That lower HTHS means FA-4 is not automatically interchangeable with CK-4, so always follow the engine maker’s chart before switching.
When you read a label that simply says engine oils, look for the API donut or the OEM approval code alongside it, not just the viscosity grade.
Standards to watch: PC-12 is coming
A new heavy-duty category, known as PC-12, is due for first licences on 1 January 2027. Expect products to appear as manufacturers finalise testing through 2026. If you buy new trucks ahead of that date, check what your OEM will allow once PC-12 hits the market, especially if you are pursuing lower-viscosity options for fuel savings.
This forward view matters for fleets standardising on heavy duty engine oils across mixed model years.
European sequences and OEM approvals
If your trucks carry Euro-spec engines, the ACEA 2022 heavy-duty sequences apply. New E8 and E11 categories replaced E6 and E9 for fresh claims from May 2023, while E4 and E7 were updated. Matching the ACEA class and any OEM numbers on the product sheet helps ensure piston cleanliness, oxidation control and aftertreatment compatibility.
Many importers in Australia refer to these when qualifying transport oils for local fleets.
Base oil quality and volatility
Two oils with the same viscosity grade can behave very differently under heat. Volatility, typically measured by the NOACK test (ASTM D5800), shows how much of the oil can evaporate at high temperature. Lower volatility helps stability and reduces top-ups, which is handy on long highway runs or when towing at gross weight. Look for NOACK numbers on data sheets and compare like-for-like.
If you are unsure how to weigh volatility against other properties, speak with experienced lubricant specialists who can interpret the full formulation, not just one figure.
Aftertreatment, SAPS and Australian fuel
Modern engines rely on DPFs, EGR and SCR. Oils with the right levels of sulphated ash, phosphorus and sulphur protect these systems while still guarding against wear. Australia has run ultra-low sulphur diesel at 10 ppm since 2009, which aligns well with the needs of aftertreatment hardware and the latest specifications. That said, duty cycle still dictates how fast soot and acids build up in your sump.
On labels and product pages, you will sometimes see engine oils described as low- or mid-SAPS. Match the SAPS level to your engine maker’s guide, particularly for vehicles with DPFs.
Viscosity choice and climate
Viscosity remains a practical decision. Many late-model on-highway engines are authorised for 10W-40 CK-4 for normal conditions, with 15W-40 still common for severe service and legacy engines. Some newer platforms permit 5W-30 FA-4 for fuel economy gains, but only if the manual or service bulletin says so. Treat a 5W-30 FA-4 and a 10W-40 CK-4 as different tools, because their HTHS targets are not the same.
Drain intervals: follow the local tables
Service intervals are not one-size-fits-all. OEMs publish region-specific schedules that factor in Australian conditions and ULSD, with separate tables for long-haul, short-haul and severe cycles. Using an approved oil to the maker’s specification is a prerequisite for extended drains; step outside those approvals and intervals usually shorten. Check your engine’s Australian schedule rather than relying on overseas figures.
Oil analysis, TBN and staying inside the window
Used oil analysis turns guesswork into data. Along with wear metals and viscosity shift, keep an eye on Total Base Number. TBN indicates the oil’s remaining ability to neutralise acids formed during combustion, and once it falls near your lab’s limit, you are better off changing early than risking corrosion. Pair TBN with oxidation and Total Acid Number to understand the full picture over your drain. Routine analysis is the smartest way to validate a product choice, especially when you are standardising engine oils across different truck makes.
A practical buying checklist
- Confirm the required API or ACEA category, plus any OEM approvals listed in your manual.
- Choose viscosity for the duty cycle, not only ambient temperature.
- Compare HTHS, volatility and published fuel economy claims within the same category and grade.
- Verify aftertreatment compatibility and SAPS level for DPF-equipped trucks.
- Align on drain intervals using the Australian tables, then back it up with oil analysis.
Apply those steps and you will narrow your shortlist fast, whether you purchase direct, through a dealer or via a distributor that specialises in truck engine oils.