Truck engine oils

It’s not easy choosing engine oil: every oil seems to be seems to have an alphabet spaghetti of specifications and accreditations on its data sheet. Then there is all marketing hyperbole. Is super-high performance oil better than ultra-high performance or vice-versa? Is long drain better than extended drain, or is it the other way round? The temptation is to default to the one thing we all understand – price per litre.

Sadly, that approach is unlikely to lead to the best answer. The other parts of the cost equation such as drain intervals, downtime, engine oil capacities, filtration costs, fuel-efficiency issues and waste oil disposal costs need to be considered. Operators with their own in-house maintenance have an excellent opportunity to explore all these elements, buying oil in bulk at the best prices and optimising the other factors to their best advantage. Truck operators with repair and maintenance contracts with dealers or other maintenance providers have less room to manoeuvre. Suffice to say that dealers’ service bills normally include a mark-up on oil, so negotiate hard.

But before getting into the economics of engine oils, it pays to get the technical aspects correct, starting by considering only those oils that comply with the vehicle manufacturer’s specifications. Anything else risks invalidating the vehicle warranty.

 

Oil specifications

Piston cleanliness is one of the main characteristics of oil conforming to ACEA’s E4 sequence.

European engine-oil specifications are based on a range of so-called “sequences” drawn up by ACEA, the vehicle manufacturers’ association.

The latest set, published in December 2012, contain four sequences for heavy-duty diesel-engines. In ascending order of performance and cost, they are:

E7

These oils suit many current engines, provided they do not have a diesel particulate filter (DPF) in their exhaust. They are invariably mineral-based and are the cheapest to buy. That helps to explain why E7 oils are the single most popular ACEA sequence in the UK heavy commercial vehicle market.

E9

These oils are broadly similar in application to E7 oils, but with one important exception – they have tight limits for sulphated ash, phosphorus and sulphur. They are dubbed low-SAPS oils and are designed specifically to suit trucks with DPF: low-SAPS oils produce less ash when they burn, so the DPF needs cleaning less frequently.  Although still mineral-based, E9 oils need a low-sulphur (group II) base oil that is more expensive than the group I base oil used for E7 oils, so expect to pay a premium for an E9.

E4

The key characteristic of E4 oils is their focus on a high level of detergency, minimising the accumulation of combustion deposits on pistons. These are not low-SAPS oils, so are not recommended for vehicles with DPF. They usually have either part- or fully-synthetic oil or the very highest-rated (group III) mineral oil as their base and so are more costly than mineral-based oils. Expect to pay around 35-40 per cent more than a typical E7 mineral oil.

E6

These top-rated oils are essentially similar to E4, but with the additional benefit of low-SAPS for vehicles with DPF. Unsurprisingly, E6 oils typically are slightly dearer than E4 oils, around 50 per cent more than our reference E7 oil.

Confusingly, truck manufacturers have their own oil specifications that sit alongside ACEA’s generic sequences. For example, many E7 oils have Mercedes’ 228.3 and Volvo VDS-3 accreditation too. Similarly, E9 oils can often meet Volvo VDS-4 and Mercedes 228.31 specifications. The list of accreditations for E4 oils often includes MAN 3277 and Mercedes 328.5. E6 oils can usually comply with MAN 3477 and Mercedes 228.51 specifications.

ACEA sequence E4 E6 E7 E9
Good for use with diesel particulate filters (DPF)? No Yes No Yes
Good for maximum drain intervals? Yes Yes No No
Fuel-efficient versions available? Yes Yes No No

Although individual ACEA sequences and manufacturers’ accreditations may pull in different directions, top-notch oils are tightly formulated to hit the small area where specifications overlap. So many E6 oils also meet E4 and E7 specifications, and have a string of manufacturer accreditations to boot. This is particularly useful in mixed fleets where there is a danger of mis-filling with the incorrect oil, but there is a price to pay for this convenience: it may prove rather costly to use expensive E6 oils in older engines that tend towards high oil consumption.

 

LOW-SAPS

 

If your truck has a DPF in the exhaust, engine-oil choice narrows to either of the two low-SAPS options, E9 or E6. One of the oil companies told us that its E9 generates around 25 per cent less ash than its E7. Its E6 oil does rather better, producing 60 per cent less ash than its E7, partly because its base oil is not so volatile as an E7’s, so less oil evaporates. This illustrates the point that not all low-SAPS oils deliver the same low-ash benefits, and with DPF ash removal costing anything from £150 to £450 for a service exchange DPF, it might be worthwhile paying more for an E6 oil rather than the cheaper E9.

 

SUMP CAPACITIES

 

It is wrong to assume that the oil costs of similarly sized vehicles will be broadly similar. For example, the 10.3-litre Cursor engine in Iveco’s Stralis takes 25 litres whereas MAN’s 10.5-litre D20 engine swallows 42 litres. Scania’s mighty 15.6-litre V8 accommodates a surprisingly modest 32 litres. Those differences have a profound on oil change costs and are not necessarily balanced by variations in drain intervals. As a rule of thumb, 7.5-tonners have a capacity of 10-15 litres; 18-tonners normally take 20-30 litres while tractor units and multi-wheel rigid vehicles typically demand 35-40 litres.

 

DRAIN INTERVALS

 

Pushing out drain intervals as far possible is a win-win-win: less oil to buy, less downtime and lower labour costs. But how far dare you go and what factors determine the decision? Most manufacturers quote maxima of around 60-70,000km for typical UK style work, reduced to 30-40,000km on tough duties like short distance urban or construction work but extended to maybe 80-100,000km on long distance and lighter duties. If struggling to decide which description best matches your operation, fuel consumption is a good yardstick: higher than average fuel consumption is indicative of tougher than average demand on the oil too.

Fuel consumption, together with other engine data such as temperature and engine load, underpins the condition-based flexible oil service indicators used by the like of Mercedes-Benz and MAN on some of their models. Using technology to fine-tune the drain interval to the particular application is undoubtedly smart but may prove rather inconvenient if the intervals vary and fail to coincide with fixed time-based safety and service schedules.

One of the main factors curtailing engine-oil life is oxidation. This is the gradual acidification of the oil; new oil is alkaline in order to combat the acidity of the combustion. Once an oil’s reserves of alkalinity are severely depleted so is its ability to resist engine wear. Taking oil into this this danger zone by delaying a change is clearly a false economy. Semi- and full-synthetic base oils are better at resisting oxidation than mineral base oils, which explains why truck manufacturers stipulate that synthetic-based E4 and E6 oils are essential if operators want to take drain intervals out to the maximum, sometimes as high as 120,000km under the right conditions.

As a rule, synthetic base oils have a higher viscosity index than mineral oils. That means they are more resistant to thinning at high temperatures and to thickening at low temperatures. They are able to retain this characteristic longer than a mineral oil with added viscosity improver. Consequently, they stay closer to their original viscosity for longer, stretching their usable life.

It is up to the operator to calculate whether longer drain intervals justify the premium payable for E4 and E6 oils, taking into account the savings on filters too. As with flexible intervals, extended drain intervals only make real sense if they delay the change by at least one scheduled safety inspection.

 

OIL SAMPLING

 

When first extending oil drain intervals it is a good idea to use oil sampling to verify how far it is safe to go. Sampling will pick up several indicators of oil life, including the degree of oxidation, as measured by the TBN (total base number). Other oil life checks include measuring soot content – soot is not only abrasive also but thickens the oil, impeding its flow – and testing for fuel dilution. Both of these will creep up as the drain period increases and they need to be kept within acceptable limits. The sampling company will advise when the oil is approaching the end of its life. A single sample typically costs around £15 or £20. It is advisable to take a series of samples to identify a trend – a single snapshot is not much to go on.

 

OIL CONSUMPTION

 

Oil consumption of around 0.075 per cent of fuel consumption is reckoned to be typical for a modern heavy-duty diesel engine.

A comprehensive approach to engine-oil management includes keeping tabs on routine, daily use of top-up oil in order to spot sick engines and to make sure that precious oil is not seeping away through leaks or pilferage. It is therefore worth knowing what constitutes normal consumption. Euro-1 engines of 15 years ago regularly would have an oil diet of between 0.1 and 0.15 per cent of their fuel consumption, even in their prime.

But burnt engine oil is a source of exhaust particulates so vehicle manufacturers have worked hard to rein-in consumption in order to comply with increasingly stringent emission limits. Healthy Euro-4 and -5 engines typically should consume oil at a rate no greater than 0.075 per cent of their fuel consumption. So, for example, if a fleet consumes 100,000 litres of fuel each month, top-up oil used in the same period should not average out at much more than 75 litres unless there is an unusually high proportion of older trucks.

 

DISPOSAL

Environmental charges levied by truck dealers as part of their service charges cover the disposal costs of waste oil and filters and may range from zero to £10.

Environmental charges levied by truck dealers as part of their service charges cover the disposal costs of waste oil and filters and may range from zero to £10.

One of the benefits of extending oil drain intervals is that you have less oil – and fewer filters – to dispose of. Some truck dealers charge for oil disposal as part of an “environmental charge” that is usually £5-£10 per service.

On the other hand, used engine-oil has a market value because it can be re-processed into fuel oil for heating, so operators with their own workshops can sell their waste oil for recycling, using the income to offset their other hazardous-waste collection costs. About 4p/litre is a typical waste oil rebate figure.

 

FUEL-EFFICIENT OILS

Operating conditions and duty cycles are a major influence on the cost-effectiveness of low-viscosity oils designed to save fuel.

Does it pay to buy expensive fuel-efficient oil that costs two to three times as much as a humble mineral oil, hoping to see a measurable reduction in the fuel bill?

The improvement in fuel economy is mostly derived from a reduction in viscosity. Thinner oil offers less viscous drag and is circulated around the engine more quickly during a cold start, reducing friction and wear. So whereas the traditional SAE viscosity rating for truck diesel engines has long been 15W-40, fuel-efficient oils are more likely to be 5W-30, featuring synthetic base oil. The 5W part of the rating indicates that the oil remains very much thinner at low temperatures, aiding cold starts; 30 indicates that it is also thinner than a 15W-40 at high engine-temperatures.

Because engine spends more of their time hot than cold, the second number in the viscosity rating has the greater influence on fuel economy. The limiting factor is that vehicle manufacturers refuse to sanction oils that are too thin at high temperatures. Thinner oils leave a thinner film on wear surfaces such as camshafts and bearings: if it is too thin the film shears. Synthetic base oils have a molecular structure that gives them superior film strength to mineral oil in these high temperature, high shear (HTHS) conditions.

This desire for acceptable engine wear protection at high temperature means ACEA sequences include an HTHS viscosity limit that effectively restricts oil to high temperature viscosity ratings no lower than 30, capping the fuel efficiency potential at typical operating temperatures. (ACEA car oil sequences have a lower HTHS viscosity limit, explaining why 5W-20 oils can be found on the shelves of Halfords). Pegging viscosity at high temperature switches the focus to low-temperature viscosity. Going lower helps here too but the net benefit naturally is dependent on ambient temperatures and the frequency of cold starts.

All that explains why the economy benefits of fuel-efficient 5W-30 oils are both limited and variable. Economy gains of 2.0 to 4.0% are normally cited: not all such oils are created equal and much depends on the baseline used for comparison. A review of the mathematical modelling and strictly controlled road testing suggests economy improvements of 3.0-4.0% are indeed feasible in urban stop-start work. Gains are highest in percentage terms when a vehicle is stationary with the engine idling because there are no aerodynamic or rolling resistance losses, so a reduction in engine losses counts for more. Low ambient temperatures and cold starts naturally enhance the benefits of thinner oil.

At the other end of the spectrum, heavier trunking operations are likely to see more modest fuel economy gains of around 1.5-2.0% when using fuel-efficient oil. An oil can do nothing to counter the growing influence of aerodynamic drag as road speed rises. Improvements of over 2.0% become more attainable as weight and average speed are reduced. Conversely, warm weather and double-shifting cuts down the number of cold-starts, limiting the benefits of even the most fuel-efficient oil.

It is important not to become hung up on percentages. The financial objective is to save money, not percentages. Shaving 1.5 % from a trunker’s fuel bill is worth more than slicing 3% off a 7.5-tonner’s, so do the sums in £. And even if it is difficult to identify such very small fuel savings, the science behind fuel-efficient oils is proven and robust, so absence of evidence is not necessarily evidence of absence. With the science sorted, it just remains for an operator to satisfy himself that the economics stack up too.

EURO-6 ENGINES AND FUTURE OILS

 

The particulate limit in the Euro-6 emissions legislation is so tight that DPF in the exhaust are inevitable. In turn, that means low-SAPS oils (E6 or E9) will be the order of day. The hunt for optimum fuel economy makes E6 with synthetic base oil likely to be the favoured choice of most truck-makers. That is the most expensive of the current ACEA sequences.

On the plus side, the two-stage NOx-reduction strategy (EGR and SCR) to be employed by most vehicle manufacturers at Euro-6 will allow them to tilt the NOx/particulates trade-off at the combustion stage to favour minimal engine-out particulates. That means less soot in the oil, so it stays cleaner and within its viscosity specification for longer. On the other hand, EGR tends to increase the soot-loading on the oil. Early indications suggest drain intervals should be broadly similar to those at Euro-5. Scania says its Euro-6 engines have a maximum intervals of 90,000km. Volvo’s first Euro-6 engine, the 460hp D13K, has a maximum interval of 100,000km, the same as its Euro-5 equivalent. Daf’s reckons its Euro-6 MX engine in the XF is good for oil drain intervals of up to 150,000km. Mercedes too is quoting a maximum of 150,000km for its OM471 and OM470 engines. Purely reliant on SCR at Euro-6, Iveco points out that the oil in its Euro-6 engines will benefit from the absence of EGR, removing the related issue of soot loading. So Iveco’s new Euro-6 Cursor engines have maximum drain intervals of 150,000km, using either E9 or E6 oils.

The presence of DPF in a Euro-6 exhaust puts even more pressure on truck-makers to minimise oil consumption in order to slow the accumulation of sooty particulates and oil-derived ash in the DPF. Mercedes says that oil consumption of its 12.8-litre OM471 Euro-6 engine should be a squeaky-clean 0.03 per cent of fuel consumption.

More generally, 15W-40 oils are expected to continue to decline gradually, giving way to wider use of 10W-40 and 5W-30 oils. Ultimately, on-going development of synthetic bases could lead to the super-thin 5W-20 or 0W-20 oils already found in the car world also proving capable of providing adequate wear protection in truck engines expected to complete a million kilometres or more. Their fuel economy benefits would be very persuasive.