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Any oil is specified by two descriptions, its viscosity and its performance levels. For any engine, the appropriate viscosity to be used depends on lowest start-up temperature and highest ambient temperature experienced, and can usually be determined from the operator’s manual. The engine manufacturer will produce a chart of recommended engine oil viscosity grades for the temperature conditions likely to be encountered.

From the chart above we can see that lower SAE “W” grades are specified for start-up at lower temperatures, and higher “non-W” grades are recommended for operation at higher ambient temperatures.  For most diesel engines operating in New Zealand, an SAE 15W-40 multigrade Diesel Engine Oil will be suitable.  This allows starting down to -20o C and continuous operation up to ambient temperatures of 40o C.  As well, SAE 10W-30 multigrade DEOs are available.  These are recommended for starting at lower temperatures and provide a slight fuel economy gain.  However, they probably shouldn’t be used for continuous operation in ambient temperatures above 30o C.

But it’s when we describe a Diesel Engine Oil’s performance that matters becomes complicated, and the use of the wrong type of DEO can damage some modern diesel engine components such as the diesel particulate filters (DPF) and exhaust treatment systems.  In the ‘good old days’ from the mid-fifties until the mid-eighties, Diesel Engine Oil performance was specified by either American Petroleum Institute (API) Service Classification API CC or API CD.  You generally put API CC oils into non-turbocharged diesel engines and API CD oils into turbocharged diesel engines.  The engine oil for Detroit 2-cycle diesel engines was determined by its ‘sulfated ash’ level, and an engine oil meeting API CC was usually used.

But, as diesel engines developed, there became a need for higher performance Diesel Engine Oils to provide better control of oil consumption, piston ring belt cleanliness, oil system sludging and piston top land deposits.  From 1985 onward, API Service classifications API CE, CF-4, CF CF-2 and CG-4 were introduced to meet increasing demands for control of high temperature piston deposits, valve train wear, oxidation resistance and soot accumulation.  Later API Diesel Engine Oil Service Classifications, such as API CH-4, CI-4, CJ-4 and the recently introduced CK-4, were developed with a focus on the requirements of Exhaust Gas Recirculation (EGR), PDF and catalyst systems in engines introduced from the early 2000s.  

The API C series of service classification are backward compatible.  API CK-4, CJ-4, CI-4 and CH-4 oils may be used where lower classification oil (such as API CG-4) is recommended.  However, at the same time that the API introduced API CK-4, they also introduced Service Classification API FA-4 which specifies SAE XW-30 DEOs specifically designed for diesel engines likely to be developed from 2017 onward.  API FA-4 oils are formulated to address greenhouse gas emissions and to meet the needs of highly developed exhaust treatment systems which have not yet been introduced.  For the first time, a new API Service classification is neither interchangeable nor backward compatible with previous classifications.  API FA-4 oils cannot be used in place of API CK-4, CJ-4, CI-4 and CH-4 oils

 API Service Classifications are specifically defined to cover the lubrication needs of North American diesel engines, though manufacturers around the world commonly use the API Service Classifications to specify Diesel Engine Oils for their engines.

In Europe, however, the European Automobile Manufacturers’ Association (ACEA - Association des Constructeurs Europeens dÁutomobiles) engine oil specifications were introduced in the early 1990s.  There are ACEA engine oil specifications for car and light van petrol and diesel engine oils (classification A/B), engine oil specifications for high performance car and light van petrol and diesel engine oils requiring catalyst compatibility (classification C) and engine oil specifications for heavy duty diesel engine oils (classification E).  ACEA classifications tend to require higher stay-in-grade viscosity performance, tougher exhaust treatment system compatibility, and some ACEA classifications are focussed on providing longer oil drain service.

 The Japanese Automotive Standards Organisation (JASO) has also developed specifications for Diesel Engine Oils which primarily focus on piston cleanliness, valve train wear, soot carrying ability and high temperature oxidation resistance.  Some of the tests used in these JASO classifications are specific to Japanese diesel engines.  The two current JASO classifications are low ash formulations for where engines are equipped with after-treatment devices such as DPFs and catalysts.  JASO DH-2 is for heavy-duty diesel engines used in trucks and buses and JASO DL−1 is for passenger car diesel engine use.

Diesel engine manufacturers themselves also became more involved with producing their own OEM Diesel Engine Oil specifications.  Caterpillar, Cummins, Detroit Diesel, Deutz, Mack, MAN, Mercedes Benz, MTU, Renault and Volvo are some of the diesel engine manufacturers who have developed their own OEM Diesel Engine Oil specifications.  Some manufacturers specify a maximum Sulfated Ash/Sulfur/Phosphorus (SAPS) level in the engine oil.  Such engine oils are referred to as ‘low SAPS’.

So, which diesel engine oil should you use?  Remember that any oil is defined by its performance level and its viscosity.  Check the manufacturer’s recommendations.  The operator’s handbook for a modern diesel engine will specify a Diesel Engine Oil meeting industry classifications such as API CJ-4 or ACEA E4/E6/E7/E9, maybe a manufacturer’s specification such as CAT ECF-3, MB 228.51 or Volvo VDS-4, and maybe also a SAPS recommendation.  An older diesel engine may need only a Diesel Engine Oil meeting API CI-4, or even API CH-4 or CG-4.  Choose the appropriate viscosity grade for the range of operating temperatures that will be encountered, and there you are.

 The best course of action is to contact your lubricant supplier and pass the alphabet soup over to them.  They should be able to recommend the right engine oil for your vehicle or fleet.  In some cases it might be best to use a couple of different Diesel Engine Oils which specifically meet the engine oil requirements of different engines in your fleet.  Commercial workshops servicing a range of diesel engine vehicles will need to carry two or even three different diesel engine oils of different performance and viscosity combinations to meet the lubricant requirements of the diesel engines they service.