Changing your car’s oil is one of the last critical maintenance tasks that owners can still do themselves, at least hypothetically. So it’s not surprising people often change their motor oil prematurely, presumably following Dad’s tried-and-true 3000-mile interval rule. But oil chemistry and engine technology have evolved in recent years and carmakers today recommend substantially longer intervals between lube jobs. Experiments with fleets have proved that extending engine oil use reduces both the total cost of ownership and its environmental impact.
Meanwhile, American motorists’ fondness for the quick-lube just continues wasting money and oil. Paradoxically, the longer change periods lead some owners to forget them all together.
Oil interval recommendations is only one of several evergreen issues and concerns that seem to bug motorists and the motor business alike. Just ask vehicle owners what weight motor oil they use and why; or if they run synthetic or dino oil. Throw in the mysteries of SAE 0W-16, API viscosity index, zinc phosphorus additives, and any such conversation is apt to tilt toward some fairly arcane matters.
“Engine oil may not be quite as exciting as some other parts of the car, but for something that is so basic, there is a pretty amazing lack of knowledge out there,” observed Eric Johnson, a veteran GM fluids and lubricants engineer. Perhaps that’s not surprising since it’s "hard to do a Consumer Reports-type test of motor oil quality and performance,” he said. Rating the rather subtle benefits of motor oil requires sophisticated, long-term measurements.
“But like everything else, engine oil is a marketed product,” he warned, “with all that entails as vendors differentiate their products.” Even seemingly clear product categories are in dispute and the terminology is obscure, so it’s caveat emptor at the auto parts store.
Engine Lube 101
A constant thin layer of oil—a slick film of hydrocarbon molecules—protects an engine’s rotating parts from heat and friction. For the most part, suppliers make motor oil from petroleum crude oil. After thousands of road miles, engine heat and pressure oxidize and degrade oil, so replacement is needed. Sensible application of the viscosity ratings and additives is key to keeping engines running at optimum.
One of the more mysterious aspects of motor oils is their grading system. In 1911 the Society of Automotive Engineers adopted the SAE J300 standard by which an oil is rated by weight, or viscosity. SAE J300 defined five grades, SAE 10 to 50, based an oil’s thickness when heated to water’s boiling point and how rapidly it flowed out of a specified size hole—faster flow got a lower rating; slower flow, a higher grade. Soon thereafter, the kinetic test was updated to use scientific units of viscosity rather than time, first in centipoise (cP), now in mPa.
In 1952 the SAE added winter (“W”) grade designations (10W to 30W), which it defined by viscosities measured at 0° F (-18° C). At freezing temperatures an SAE 20 oil refined from black Gulf of Mexico crude was much thicker than an SAE 20 made from amber Pennsylvania crude. This difference came to be measured with a viscosity ratio metric called the viscosity index—the change in viscosity with temperature. The less a motor oil’s behavior changes from hot to cold, the higher its viscosity index, which is marked with a “W”. A 5W-30 motor oil performs like a SAE 5 motor oil would in the winter cold, but delivers SAE 30 viscosity at engine operating temperature.
Oil viscosities have been falling in recent years. Not long ago, SAE adopted SAE 16 as a lower viscosity standard to avoid customer confusion with the “W” grades of oil, that is, SAE 16 follows SAE 20 instead of SAE 15 to set a precedent for future grades whereby they count down by fours instead of fives: SAE 16 down to SAE 4.
“Companies such as Honda are using 0W-16 oils on smaller, tighter engines to help improve fuel economy,” said Roger Hood, U.S. motorsports technical advisor for ExxonMobil.
An engine needs oil that is thin enough for low-friction cold starts, but thick enough for protection when the engine is hot, Johnson said. Since oil gets thinner when heated, and thicker when cooled, multi-grade, or multi-viscosity oils are commonly used that operate at a range of temperatures. Maintaining the oil hydraulic performance is another is prerequisite for such things as operating engine valves and minimizing pumping losses.
Today’s motor oils blend base oils (75 to 80%) which are usually made from petroleum-based hydrocarbons, though a growing market niche for synthetic base oil has developed in recent years, according to the Society of Tribologists and Lubrication Engineers. The American Petroleum Institute classifies base oils into five groups according to sulfur and saturated (waxy) paraffin content as well as viscosity and pour point. Group I base oils have lower levels whereas Group II and III base oils have higher levels.
Multigrade oils include special additives, perhaps 5% by weight, to boost viscosity at elevated temperatures, keep engines clean and corrosion-free while delivering less friction and wear. They can include viscosity index improvers (5 to 10% of the additives), rust and corrosion inhibitors (<1%) and additive packages (10 to 20%) containing dispersants, friction modifiers, detergents, surfactants, anti-oxidants, metal deactivators, sealants, pour-point depressants, anti-foaming agents and more.
“It’s important to see that these additives are activated at different temperatures, which is why we stress component blending,” said David Tsurusaki, strategic global alliance manager for ExxonMobil. “It means that each of the additives work in conjunction with the others at the right time.”
In recent decades, regulatory limits have been placed on the use of ZDDP, or zinc dialkyldithiophosphate, a popular sacrificial anti-wear additive with added oxidation- and corrosion-resistance properties. Unfortunately, ZDDP which plates as a phosphate glass out on contact surfaces, is environmentally toxic and harms exhaust catalysts.
Synthetic motor oils—a segment pioneered decades ago by Mobil 1—are custom-designed to offer greater tolerance to heat and oxidative aging, so they can probably lengthen the lives of most engines, especially those running at higher loads, such as when towing a trailer. But along with synthetics’ documented benefits come high costs, so in some quarters their ultimate utility remains in debate.
Whereas mineral base stocks are complex mixtures of a range of naturally occurring hydrocarbons and paraffins, synthetic base oils are just that, fully man-made. Synthetics are made by polymerizing small hydrocarbon precursors into large molecules that are uniform in structure to guarantee predictable properties. They're “soft” enough to retain good viscosity when hot, yet with branched structures that impede solidification and so allow flow when cold. Synthetics have less need for viscosity index improvers, which is most vulnerable to thermal and mechanical degradation.
Synthetics were first produced by German scientists during World War II, when vehicle and aircraft mechanics at the Russian front found that they stayed fluid in sub-zero temperatures. Decades of subsequent use lubricating hot-running aviation engines and then race cars followed when in the mid-1970s automotive synthetic motor oils appeared.
“The racing community has always been step ahead in adopting new oil technology,” Mobil 1’s Hood said. “Synthetic oil gives teams more power, but engineers can use that advantage for better efficiency, too.”
The API grades synthetics as Group IV. They were traditionally made from chemical reactions such as polyalphaolefins, with additions slippery esters, but in time competitors developed hydroprocessed synthetics founded on highly refined mineral base oils derived from Group III stocks. These products bridge the gap to traditional synthetics by providing similar performance at lower cost. But such claims are always a matter of debate.
“Except in Germany where the original definitions remain, synthetic oil is now just another marketing term,” Mobil 1’s Tsurusaki said. He noted that nowadays it’s hard to say exactly what synthetic oil is, “though we know what we think it is.”
And then there are the synthetic blends—a bridge to a bridge, maybe?