Engine oil decisions used to be simple: synthetic lubes cost more than mineral-based oils, but they offered tempting performance advantages, including fuel savings. If you had to drain the lube, though, the economics became more difficult to figure. Synthetics still performed better, especially in low temperatures, but not enough to cover the added cost for the synthetic product if you had to throw it away regularly.

But things have changed. The price premium for synthetics isn’t as great as it once was, and fuel costs are way up. Shop rates are higher, technicians are harder to find, and uptime is more valuable. Score one for synthetics at your mental cash register.

At the same time, mineral base stocks for lube oils are better, too. Hydrotreated, highly saturated base stocks are more stable oxidatively and thermally, and they form a good base for much more complex additive packages. Score one for minerals.

Let the debate begin.

One camp-led by a major supplier of synthetics, admittedly-says the value of synthetic lubricants has never been better. As always, they allow significantly extended drains. And, yes, synthetics do offer fuel economy gains in today’s cost-conscious environment. Synthetics are a no-brainer for all heavy-duty lube applications, they say.

The other side, led by a major supplier of hydrotreating technology and saturated oil stocks, says the performance gap between synthetics and new-technology mineral oils is negligible. The difference in price, however, is not. And by the time the additive package is refined, the argument goes, mineral-based products can perform just as well as synthetics in any case.

There’s maybe a third camp that agrees with both and offers a blended product that combines the benefits of synthetic’s viscosity stability and the cost savings of mineral lubes.
Adding to the confusion is a lack of consensus at the marketing level about just what is-and isn’t-a synthetic. At its simplest, a synthetic base-stock lubricant is one that is manufactured by a chemical plant. As Shell defines it, synthetics are a “man-made (not naturally occurring) base oil.” Most often used in engine and gear lubricants, synthetic base stocks are poly-alfa-olefins (PAOs), with esters and other hydrocarbon compounds also made as synthetic lubricants. They are petrochemicals with a well-defined molecular structure and equally well-defined physical properties.

In contrast, mineral base stocks-the basis for conventional single and multigrade lubes-are oils based on crude that comes out of the ground. These mineral oils tend to be a mixture of different hydrocarbons with varying amount of unsaturated molecules like aromatics. They are subject to chemical reaction including oxidation, which causes them to change viscosity and thicken over time and temperature. With technological advances since the mid-1980s, it’s now possible to stabilize mineral base stocks and convert them into more predictable molecules. The hydrotreating process-as opposed to conventional solvent-based extraction-in refining plants breaks open the unsaturated bonds in the molecules, substituting hydrogen and reducing the number of available oxidatable sites.

Petro-Canada chemist Luc Girard compares those “predictable” molecules to Lego blocks that are built up until the desired viscosity is reached. Before hydrotreating, the molecular structure of a mineral base stock is nowhere near as regular and therefore resists chemical manipulation by way of additives.

In terms of definition, the solvent-extracted mineral base stocks are classified in five groups, based on the amount of treatment and processing they receive (see sidebar, next page). The classifications range from Group I (basically untreated mineral base oil) to Group IV (pure synthetic PAOs). All the other lubricant stocks not covered by these classifications, such as the esters, are in Group V.

Despite these clear technical definitions, marketers referring to hydrotreated mineral stocks as either synthetic or semi-synthetic have muddied the minds of consumers.

The hydrotreating process significantly changes the physical and chemical properties of the base oil. Removing the aromatics takes out the available sites for oxygen reaction, greatly improving the viscosity stability of the base stocks and their ability to extend drains. The changes and chemical manipulations are substantial enough, says Petro-Can’s Girard, that the “synthetic” label is justified in his own company’s “Duron” product. Those manipulations include a de-waxing process called “hydroIsomerization,” as opposed to traditional “chill” de-waxing, which is said to improve low-temperature fluidity or pour point dramatically so that it compares favorably with classic PAO synthetics. And that, as much as any other quality, he says, defines Duron as “synthetic.”

Alternatively, also drawing on the marketability of the synthetic name, products that are a mixture of pure PAO synthetic with basic Group I and II mineral stocks are sometimes referred to as “semi-synthetic.” In the latter case, incidentally, there is no definition of how much of the mixture has to be synthetic to qualify it as a semi-synthetic.

While the base stocks affect the performance, cost, and quality of the lubricant, the performance of any oil is determined more by the additive package blended in to the final product. High torque, high temperatures, and rigorous emissions requirements subject the iron in powertrain components to extremes, and today’s lubricants play a vital role in their reliability and durability. In engines, for instance, current emissions regulations require retarded injection timing, which loads up the oil with soot. To prevent the microscopic particles of carbon getting together and forming abrasive nodules, engine oils have dispersants in the additive package. They’re so good that when you do an oil change, the lubricant might have as much as a gallon of carbon suspended in the hot black stuff that drains from the engine’s pan.

Additives also include anti-foaming agents, anti-oxidants, corrosion inhibitors, and a host of other improvers that modify the base oil’s performance. And because they perform specific functions, the additives get used up, or they load up, as is the case with dispersants. Either way, they must be replenished through added oil, or discarded and replaced. One point worth considering: because synthetics are an expensive base and the end user expects to pay more, they may have a more expensive additive package which will permit a longer drain interval.

Additives are critical to gear oils, too. This is why there’s a differentiation between lubes for axles and transmissions. The chemistry that produces a good extreme pressure additive is generally based on sulfur. But the “bright” metals found in transmissions, such as the bronze in synchronizer rings, are attacked by these same additives that protect the axle pinion and ring gear.
The upgrading of mineral base stocks and the use of more effective additives means that synthetics may no longer have a clear performance advantage.

According to Ross Iwamoto, product development scientist for 76 Lubricants, fuel economy gains from using synthetics throughout the powertrain are potentially available. But, he says, there’s a much bigger gain, as far as engine lubes are concerned, to be had from switching to a different viscosity grade. Iwamoto points out that diesel engines, because of their robust construction, are about 97% hydrodynamically lubricated-quite different from a spark-ignition engine. So while the oil itself has to be pumped, and at low temperatures synthetics have a distinct advantage, the “slipperiness” of the lube base isn’t really a factor in fuel consumption.
Mike Ragomo, product advisor for ExxonMobil, adds both laboratory and real world fleet testing show at least a 3% gain in fuel economy from switching to synthetic lubes when used throughout the powertrain. However, the ExxonMobil commercial synthetic is Delvac 1, an oil with a 5W-40 viscosity rating.

An SAE paper (2000-01-1993) given by some of Ragomo’s colleagues a year ago addresses the performance qualities of synthetics, including extended drains and fuel economy. It says the fuel savings of synthetics are well documented, and goes on to show that a 5W-40 synthetic allows up to a 3% economy enhancement over a 15W-40 mineral oil with both lubricants fresh. Significantly, with a heavily contaminated synthetic loaded with 7.3% soot, there are still gains over fresh mineral oil. Given that mineral oils thicken with soot loading, this paper would indicate that synthetics should deliver at least 3% gains in drains to 80,000 to 100,000 miles.

Ragomo is a chemist and an enthusiastic booster for synthetics. Like his colleagues, he maintains the economy gains are there, especially at extended mileages. But it’s the ability to stretch those drains that really argues for synthetic base stocks. He says the payoff for drains to 80,000 miles is the reduced frequency of oil changes, saving shop time of an hour per truck per oil change. The fuel economy is merely a side benefit.
Sounds convincing, but the widely respected Jim McGeehan, manager of engine oil technology at Chevron, co-authored an SAE paper (2001-01-1968) that disputes the ExxonMobil findings. The paper says the fuel economy difference between a 5W-40 PAO/ester synthetic and Group II 15W-40 mineral oil is too small to measure, and concludes that testing could find no statistically measurable differences between a well formulated Group II-based lubricant and one based on a PAO/ester synthetic.

The Chevron paper compares the differing base stocks in extended drain testing over 1995 Caterpillar and 1996 Detroit Diesel power test fleets. It also includes data from 1999 Cummins N-14 tests-significant because post-1999 engines present the toughest challenge to lube oils in coping with soot. The paper concludes that “premium additive packages using API Group II base stocks at greater than 99% saturate levels can equal synthetic oils in fuel economy and extended oil drain capability.”

That’s at odds with the ExxonMobil findings, though the definition of extended drain is somewhat different. The data points in the Chevron study were taken at 15,000-, 30,000-, 45,000-, and 60,000-mile drains. Ragomo’s contention is that using synthetics with the premium additives in the ExxonMobil lubricant offers the potential to extend drains out beyond this point. And the longer the drain interval, the more productive the vehicle can be.

That’s a key benefit to using synthetics in the driveline. Eaton/Dana and ArvinMeritor both offer an extended 750,000-mile warranty for transmissions and axles with synthetic gear lubes.

But even the synthetics’ seeming unassailable position in these components is under storm from Group II base stocks. Chevron has gained approval from Eaton for 250,000-mile transmission drains and the full warranty when fleets use its ESI gear lubes. The axle drain intervals are also 250,000 miles for Eaton, though the ArvinMeritor axles go to 500,000 miles intervals with the Chevron product. And now Pennzoil is in the act, so far only with approval from Dana for 250,000 miles in axles (the approval process is under way at ArvinMeritor).

According to Pennzoil gear lube product manager Lisa Silva, the 250,000-mile approval is an excellent start. She says a fleet that trades at three years will specify the synthetic and never change transmission or axles lubes. Fleets that run the trucks until the drivetrain warranty is all but expired are a natural for a 250,000-mile drain product like Pennzoil’s. It runs the first half-million miles with synthetic, then, as a money saver, spins out the rest of the warranty period with a mineral-oil, 250,000-mile change product.
She’s looking, by the way, for 500,000-mile drain approval.

The lube cost savings can be significant for a maintenance department that has the right trade cycle, says Silva. Those savings are also available with the similar extended intervals currently approved for Chevron’s borate-chemistry gear lubricants.

Chevron, though, is looking for parity with the synthetic gear oils currently approved by Eaton and ArvinMeritor for 500,000-mile drains. Chevron says it has evidence that it can meet those durability targets.

It remains a fact that synthetic base stocks are more expensive than Group I or even the hydrotreated Group II and III oils. According to Citgo’s Betner, there are alternatives that deliver almost all of a synthetic’s benefits but at a lower price, citing his company’s Para-Syn, a blend of Group IV, Group II, and Group I base stocks. He says this product delivers 95% of a synthetic’s performance for around half the price.
He concludes, though, that a fleet is best served by deciding what its objectives are and then crafting an oil-change program around those objectives. Synthetics can save the right operation big time, it would appear. But so can premium non-synthetics and semi-synthetics.

Fact is, at least, these days it’s hard to go wrong with whatever choice you make.