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What’s that Stuff

What's that stuff? Motor Oil

Lubricant keeps car engines from clanging apart, sparks debates on how often it should be changed

by Stephen K. Ritter
March 13, 2006 | A version of this story appeared in Volume 84, Issue 11

Credit: Jose Luis Pelaez/Photodisc
Credit: Jose Luis Pelaez/Photodisc

Do you know where your car's dipstick is? If the answer is yes, do you recall when you last looked at it?

In bygone days, people took time to regularly check the level of motor oil in their car and faithfully changed the oil and filter every 3,000 miles. But now, with new engine designs that require less maintenance, improved motor oils, and ubiquitous 15-minute oil-change garages, many people have dropped the routine of popping the hood to check the oil or to change it themselves.

Why motor oil? The viscous fluid spurts out of strategically located holes in the engine or splashes off moving parts to form a thin film on the metal surfaces. The primary role of motor oil is to reduce friction and prevent corrosion. But oil also serves to dissipate heat and to hold in suspension the micrometer-sized by-products of engine wear (metallic particles), combustion (soot), and oil degradation products.

But what is motor oil? Simply put, it's about 90% paraffinic (heavy) hydrocarbon base stock distilled from crude oil, with the remainder constituting the "additives package," notes Dennis L. Bachelder of the American Petroleum Institute (API). The lengths of the hydrocarbon chains vary depending on the desired properties of the oil, but base stocks generally contain an assortment of linear and branched compounds in the C16 to C50 range.

Base stocks are divided into five groups, Bachelder notes. Groups I-III are basic crude oil fractions, with little extra refining done to Group I and some cracking done for Group III. Groups IV and V are called "synthetic" oils because they have been subjected to several synthetic refinery processes. Group IV is made up exclusively of poly(α-olefin)s, while Group V is a catchall group that includes polyol esters and polyalkylene glycols, he says.

Specifications and certifications are important when it comes to motor oil, and an array of national and international organizations set standards and perform evaluations. API's ratings include evolving oil performance specifications for gasoline engines (currently denoted as SM) and diesel engines (currently CI-4).

The grades of oil are based on viscosity standards set by the Society of Automotive Engineers (SAE). Oil grades are based on a viscosity range at a standard temperature; the higher the viscosity, the higher the SAE grade number. These numbers, which range from 0 to 50 or higher, are referred to as the "weight" of motor oil.

The oil's cold-weather weight is indicated by the letter "W," meaning "winter," as in 10W, which is gauged by the oil's viscosity at 0 °F. The warm-weather weight is based on the viscosity measured at 210 °F. Oils used to be sold mostly as single-grade products, and still are for some special uses, such as extreme weather conditions or for racing cars. But the viscosity range of single-grade oils is too limited for general use. That's where multigrade oils come in.

High-molecular-weight polymers (viscosity index improvers) such as poly(methyl methacrylate) and ethylene-propylene copolymer are added to a low-viscosity oil base stock to create multigrade oils that work through thick and thin. At cold temperatures, the rubberlike polymer molecules exist as balled-up coils and don't thicken the oil significantly, Bachelder says. But at warmer temperatures, they expand to more linear random coils to prevent oil from thinning out too much. Thus, common multigrade monikers-indicated by two grade numbers-are 5W-30 for colder climates (falling below 0 °F), 10W-30 for intermediate climates (down to 0 °F), and 20W-50 for warmer climates (down to 32 °F).

Now to the additives. One major class of motor oil additives is metal phenoxides, such as RC6H4ONa, where R is an alkyl chain. These compounds play several roles, including acting as bases to neutralize acids that form from sulfur compounds in the oil and to prevent hydrocarbon oxidation, which can lead to sludge formation. The phenoxides and their sulfate and carboxylate analogs also serve as detergents to help solubilize or suspend soot and to carry particulates to the oil filter to be removed from the oil stream.

Another key additive class is antiwear agents, such as zinc dialkyldithiophosphates, Zn[S2P(OR)2]2, where R is a linear or branched alkyl group. These compounds form a micrometer-thick, cross-linked barrier layer on metal surfaces under high pressure to protect against scuffing. The zinc compounds and various amines, such as diphenylamine, also serve as corrosion inhibitors and antioxidants.

As with other types of consumer products, the plethora of motor oil additives for sale at your local auto parts store vary from maybe being useful to being snake oil. One can assume that if an additive were useful, it would already be in the oil, or it would carry an endorsement from an oil company.

Finally, the burning question about motor oil: How often should it be changed? Conventional wisdom has held that the oil should be changed about every 3,000 miles. This notion has been ingrained into people's heads for decades, in part as a marketing ploy by oil companies. The 3,000-mile interval made sense when engines used single-grade nondetergent oils. But with the latest oils and car designs, it's no longer necessary to change oil that often under normal driving conditions.

"You can change your oil every 3,000 miles if you're really particular, but we recommend changing it every 5,000 miles," commented Tom and Ray Magliozzi, the erudite grease-monkey hosts of the popular National Public Radio program "Car Talk," on a show last year. "If you change it more often, you are just wasting your money and creating a disposal problem," they noted. They are considering revising their recommendation to 7,500 miles. Synthetic oils, because of their better properties, need to be changed less often, at intervals up to 25,000 miles or more. In the end, the best advice on the type of oil to use and the frequency of oil changes is to follow the manufacturer's recommendation in your car owner's manual.



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