This scenario allows the researcher to test prototype oils versus known reference engine oils in near-identical vehicles with near-identical engines operating under near-identical driving conditions. These similarities are an attempt to reduce variability in the test matrix so that the oil differences are more apparent.

While this practice does have merit, one study of used engine oils initiated a few years ago tried to approach the testing from a slightly different perspective. Shell Global Solutions (US) Inc. chose to evaluate the used engine oils from everyday passenger vehicles driving everyday cycles in the Houston, Texas, area.

The company collected 457 samples of used SAE 10W-30 engine oils from passenger cars and light trucks that were found at local oil change facilities in the Houston area, and the company also initiated an employee fleet test, using employee vehicles driving their same daily cycles. The sample collection included several oil brands containing conventional mineral oil base stocks, synthetic base stocks and a combination of the two.

As detailed in SAE paper 2003-01-1957 (JSAE paper 20030339), the final collection of used engine oils was cataloged and then submitted for a battery of laboratory tests. Given the time frame of the study, remember that the respective oil brand manufacturers originally identified the majority of these oils as GF-2 and a small quantity as GF-3.

The intent of this project was to research the condition of the used engine oil as it existed in many commuters daily driving cars and light trucks. Though automotive manufacturers have recommendations for oil drain intervals, many consumers seem to be confused about how the definitions apply to them personally.

A study performed by Harris Interactive on behalf of Pennzoil-Quaker State Co. in 2001 learned that of 3,345 Americans polled online, more than 80 percent of the respondents initially classified themselves under normal driving service. However, after reviewing the definitions, more than half of the same respondents correctly identified themselves under severe driving service.

The 457 used oil samples were submitted for several laboratory tests to gauge the condition of the engine oil that existed in the test vehicles at the various intervals of driving miles. The lab tests explored the base stock degradation, additive depletion, oxidative stability, acidity/acid control and viscosity.

To begin, as we stated, the oil samples were identified as SAE 10W-30 engine oils. The kinematic viscosity results are shown in Figure 1. From looking at those samples with the least amount of miles, the viscosity of the engine oils appears to be within the new-oil viscosity specification for an SAE 10W-30 engine oil, of 9.3 to 12.5 centiStokes at 100 degrees C.

However, the cluster of data points appears to dip slightly during the first miles of driving. The most likely explanation for this dip is due to some potential shearing of the viscosity modifier additive, which helps some multi-grade engine oils span so many SAE viscosity grades.

Viscosity modifiers are typically long molecules that are sometimes subject to mechanical shearing by the engines oil pump, valvetrain and other hardware. Once these long molecules are sheared into smaller molecules, the viscosity of the engine oil may correspondingly drop. This is sometimes referred to as viscosity breakdown. In fact, in some cases the data points in Figure 1 indicate that the engine oils with relatively low accumulation of miles were below the SAE 10W-30 viscosity grade limits.

Looking further to the right in the same figure, the data tend to creep upwards as miles accumulate, indicating that the oil begins to thicken in viscosity. To better theorize why, lets shift the focus momentarily to other additives in the engine oil.

A typical motor oil formulation may contain multiple antioxidant additives to help protect the base stock from excessive heat. Such hot spots within the oils circulation are located near the piston rings and the combustion chambers. In such areas, the oil may see temperatures above 400 degrees F.

When the used oil samples from this study were evaluated to determine the level of remaining antioxidant additive effectiveness, the results were quite clear. Some of the antioxidant additives were spent within the first 2,000 miles of oil life while others were spent within 3,000 to 5,000 miles.

Once the antioxidant additives are spent, the base stock is more vulnerable to oxidation. Referring back to Figure 1, which details the viscosity of the oil as more miles are driven, this fact regarding the antioxidants depleting helps explain why the viscosity of the oil tends to increase as more miles are accumulated.

Though conventional mineral oil base stocks can indeed be used in engine oils capable of working in this hot environment, synthetic base stocks may offer benefits above conventional mineral base stocks due to their inherently better oxidation resistance.

In Figure 2, the degree of oxidation occurring in the base stocks is confirmed.

But bear in mind that high temperatures are only one type of enemy of engine oil. Naturally occurring contaminants can enter the oil and limit its life as well.

For example, water. It does not sound like much at first but relax your mind a little deeper. As gasoline is combusted in an engine, a naturally occurring byproduct is water. This applies not only to historic collectable vehicles but also to the latest gasoline engines with the most modern electronics.

Since the combustion temperatures are well above 100 degrees C, it is hoped that the naturally occurring moisture simply exits the exhaust as steam. However, nothing is ever quite as simple as it should be.

As a result of the engines piston/cylinder fit being less than 100 percent efficient, some of the blow-by products of combustion (including water and partially burned gasoline) slip past the piston rings into the dead air-space of the crankcase – just above where the oil collects.

The condensation of partially combusted gasoline or oxides of nitrogen or sulfur can lead to an accumulation of acid in the engine oil. And referring back briefly to the oxidation discussions above, partial oxidation of the base stocks can also contribute to acids. The result is that acid is a naturally occurring contaminant of the engine oil that occurs whenever the engine is running.

Engine oil formulators know to expect acidic contaminants and combat these by carefully balancing other additives into the engine oil to buffer the acid. But since the acids are continually being produced and slowly contaminating the engine oil, eventually the buffer will become overwhelmed if left unchecked. Figure 3 superimposes the buffer value called TBN (Total Base Number) on top of the TAN (Total Acid Number).

Without getting deeper into details, a few observations and theories can be pulled from this collection of data.

Oxidation is a fundamental part of engine oil aging. The hostile engine environment supports chemical reactions that are catalyzed by metals and accelerated by heat.

Certain additives within the engine oil are sacrificial in nature and hence need occasional replenishing.

Naturally occurring contaminants are ever-producing during engine operation.

Engine oil formulators must carefully balance several additives, base stocks and viscosity modifiers as a total package. No single additive can be heavily overtreated without potentially impacting another part of the engine.

While synthetic base stocks may offer added protection in some regards, one must remember that driving cycles vary greatly from consumer to consumer and each driving cycle may stress a different part of the engine oil.

Average consumers may not have access to such chemical analysis of their engine oil and thus must fully understand their vehicle manufacturers recommendation for oil drain intervals.

The author acknowledges that engine hardware has advanced and engine oil technology/specifications have also advanced since this study was conducted. Furthermore, this study focused on one geographic climate, which may differ elsewhere. However, at the same time, it does reinforce that daily drivers may see unique driving conditions that have various effects on limiting their oil drain interval. The driver is foremost encouraged to fully understand and heed the oil change recommendations provided by the vehicle manufacturer, else heed the side of caution.