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GM Oil Life System

General Motors has made it official...on its 2004 and newer police vehicles, do not change the oil every 3,000 miles. According to the owner’s manual, wait for the proprietary, algorithm-based Oil Life System (OLS) to illuminate the “Change Engine Oil” light in the message center of the dash.

Depending on driving conditions, even in police “severe service” use, oil change intervals may be safely extended up to 7,000 miles. That means buying less oil, buying fewer filters, disposing of less oil, less shop labor to change oil and less vehicle downtime. And all this comes without risking the reliability or longevity of the engine and in full compliance with the factory warranty.

The 3,000-mile oil change figure dates back to 1968. Back then, it was both a conservative number and based on engine designs and oil technology at the time. Both have changed radically in the past 35 years. The OLS was first installed on a very limited number of 1988 model cars.

While GM has had Oil Life System software on 20 million cars built since 1995, this was not formalized into a Simplified Maintenance Schedule until the 2004 model year. GM vehicles built before 2004 should continue to follow the 3,000-mile oil change intervals. GM vehicles built in 2004 and after should follow the Simplified Maintenance Schedule, i.e., follow the oil message dash light.

The OLS has a “grace period” of 600 miles, i.e., change the oil within 600 miles of the light coming on. In performing an oil analysis on vehicles with OLS, GM found, in all cases, the OLS signaled the need for an oil change before the oil was worn out. This was true even for oil with 10,000 to 14,000 miles.

In some cases, the type of driving will result in the same oil change interval as in the past. Short trips, where the engine oil is below operating temperature, or under heavy loads, where the engine oil is above operating temperatures, both shorten the oil change interval. Long trips or highway driving both lengthen the oil change interval.

Traffic units driven fast, pushed hard, and then idled may not see extensions much beyond the long-recommended 3,000-mile period. However, many patrol cars and virtually all admin cars will see a doubling or tripling of the miles between oil changes.

On short trips (less than 2 miles) the engine oil does not get up to the ideal temperature of 212 degrees Fahrenheit. In this worst-case scenario, the drain intervals probably will be between 3,000 and 4,000 miles.

In typical urban driving or typical mixed driving, the interval probably will be extended to between 4,000 and 6,000 miles. Even vehicles used for trailer towing can expect to go between 5,000 and 7,000 miles between oil changes. Cars, pickups and SUVs used for highway driving may go between 7,000 and 12,000 miles. The GM Oil Life System may extend petroleum-based oils into the range of much higher-priced synthetic oils.

Rpm and Oil Temp

The OLS does not sense or monitor the condition of the oil. And the OLS is not indexed to time. And it is not based on miles. In fact, if the GM OLS indicator does not come on, the default recommendation is to change the oil once a year or at 12,000 miles. The Oil Life System does not measure viscosity or acid content, although such on-board, real-time sensors are available or under development. Finally, the Oil Life System is not an oil level indicator, although many GM engines use a separate oil level sensor.

The GM Oil Life System is a computer-based algorithm based on two primary factors: engine rpm and oil temperature. GM has established a maximum allowable number of engine revs, totally independent of both time and mileage.

An engine operated at 3,000 to 4,000 rpm will need the oil changed sooner than an engine operated at 2,000 to 3,000 rpm, all else equal. An engine that idles for hours at 750 rpm, but puts zero miles on the odometer, is still factored into the oil change interval.

The number of revs, however, is factored by the oil temperature during those engine revs. GM determined five different penalty factors based on oil temperature. The ideal oil temperature for gasoline-powered engine is between 165 deg F and 255 deg F. Oil temperatures both colder and hotter than this range have a penalty factor. For example, oil at 120 deg F and at 285 deg F have the same penalty factor. Oil at -5 deg F and at 300 deg F both have a larger penalty factor. Engine oil above 320 deg F is severely penalized.

The engine revs are multiplied by the oil temperature penalty factor. Then that weighted or corrected number is subtracted from the maximum allowed engine revs. Note that this is oil temperature and not coolant temperature.

The GM Oil Life System is based on the kind of engines used in law enforcement, i.e., gasoline-powered, 3.8L V-6 engines in mid-size cars and 5.3L V-8 engines in pickups and SUVs. Each engine has an algorithm developed specifically for that engine. Each OLS computer algorithm is engine specific because each engine behaves differently under various driving situations. The OLS for the 3.8L V-6 in the Impala is different than the OLS for the 5.3L V-8 in the Tahoe.

Oxidation Stability

During the development of the OLS algorithm, four different oil characteristics were measured to determine the extent of oil deterioration, 1) oxidation stability, 2) acidity, 3) alkalinity and 4) viscosity change.

Engine oil is made up of the base oil stock and an additive package. The working life of the oil depends on both the base oil and the additives, and also the engine and its operating conditions. The function of the oil is to lubricate, clean and cool the engine. In fact, 40% of the engine cooling comes from the oil. The additives interact with both the oil contaminants and the byproducts of the oxidation caused degradation to minimize the effect of contaminants and byproducts on the oil.

During use, engine oil changes chemically because of 1) oxidation and 2) contamination with fuel, water, ethylene glycol, worn metal and the soot byproducts of combustion. The additives such as antioxidants, anti-wear, corrosion inhibitors, detergents, dispersants and viscosity modifiers, improve the long-term stability of the oil. Antioxidants (obviously) minimize the rate of oxidation when the oil is exposed to high temperatures.

Oxidation Stability (thermal stability) is the chemical integrity of the oil after exposure to engine operating temperatures. Of the four factors evaluated, oxidation stability was the most critical. In other words, the engine oil generally went outside the parameters of oxidation stability before acidity, alkalinity or viscosity change became limiting factors.

Oils with poor thermal and oxidative stability break down rapidly at operating temperatures and especially high temperatures. Oil that breaks down loses the ability to lubricate and protect the engine. And the byproducts of the breakdown form sludge, varnish and acids.

Oil integrity, and the limiting factor for oil change intervals, is oxidative stability.

Acids and Bases

Also part of the OLS algorithm were the Total Acidity Number (TAN) and the alkalinity expressed as the Total Base Number (TBN). Engine oil breakdown is closely related to the level of acidity. As oxidative degradation increases, the acidity of the oil increases and the TAN increases. Conversely, as antioxidants, dispersants and detergents degrade, the oil becomes less alkaline and the TBN decreases.

The TAN is the measure of both the weak organic acids and the strong inorganic acids in the oil. In gasoline-powered engines especially, elevated operating temperatures can generate high levels of weak organic acids. A high TAN will cause the formation of gums and lacquers on metal surfaces, and when mixed with water, it will cause corrosion.

TAN measures the breakdown of the base oil stock. TBN measures the breakdown of the additive package, and its ability to neutralize acids. In short, the TBN measures the amount of active (usable) additives remaining in the oil.

The TAN is a measure of the amount of acidic substances absorbed. In gasoline engines, complex peroxy acids are formed, and these attack the TBN. Engine oil can have both acid and base compounds at the same time, i.e., the TAN and TBN don’t necessarily relate to one another, nor is one the opposite of the other.

Strong acids attack piston rings and cylinder walls. Weak acids attack lead-based bearings. Inorganic acids, such as sulfuric and nitric from the combustion and hydrochloric from the byproducts and contaminants, break down the additive package. Organic acids, such as carboxylic acids, result from the fuel and oil oxidation. Sulphuric acid as a result of sulphur in the fuel and is more of a problem with diesel engines than gas engines. Lubricant alkalinity is one of the key factors that governs oil life in diesel engines.


Viscosity is the oil’s resistance to flow with respect to temperature. The Viscosity Index is a measure of oil resistance to thinning as the oil gets very hot, because a thick enough layer of oil is needed to prevent wear. It is also a measure of the oil’s ability to flow as the oil becomes very cold, because this affects pumpability. Modern engine oils have a viscosity rating such as 5W-20. The “W” stands for “winter” not “weight.”

The multigrade 5W-20 used in the Ford CVPI acts like a 5-weight oil under cold temperatures and flows like a 20-weight oil under hot temperatures. Using a lighter oil than recommended may not provide adequate engine protection. Using a heavier oil than recommended will reduce fuel economy, reduce engine performance and increase oil temperatures in critical areas like the bearings.

The Viscosity Index is increased (the oil acts thicker) because of oxidation, foaming or mixing with water, soot or solid contaminants. Water produces emulsions that increase the viscosity and decrease the load-bearing ability of the oil. Water is a major cause of increased wear, internal corrosion and sludge build-up. The Viscosity Index is decreased (the oil acts thinner) because of fuel or solvent decontamination, molecular shearing, or contamination by refrigerant.

The higher the temperature, the lower the viscosity and the thinner the oil becomes. The lower the temperature, the higher the viscosity, and the thicker the oil. For that reason, the Society of Automotive Engineers (SAE) establishes oil performance at a number of different temperatures.

The measurements for the Viscosity Index are taken at two temperatures representative of the oil in the oil pan of a warm engine, 105 deg F and 212 deg F. This is called kinematic viscosity testing. In another test, the cold cranking and cold pumping tests for 5-weight oil are conducted at -20 deg F. The minimum kinematic viscosity is done at 212 deg F. The kinematic viscosity, minimum and maximum, for 20-weight oil is done at 212 deg F, and the high-temperature, high-shear tests are done at 300 deg F.

High-temperature, high-shear rate, determines the ability of the oil to flow in narrow spaces in warm engines. This has important implications for both valve and bearing wear, and also for fuel economy. One of the reasons Ford recently changed from 5W-30 oil to 5W-20 oil was to reduce the friction caused by the oil and improve fuel economy. As a general rule, synthetic oils have a better shear stability, or shear resistance, than petroleum-based oils.

Low viscosity oils flow better than high viscosity oils and are easier to pump. They circulate faster through the oil galleries. Low viscosity oils have a lower oil pressure, but the oil pump delivers a greater volume of oil. Since oil does not compress well, a lower oil pressure results in a lower oil temperature from the pumping and oil passages.

The key to multiweight oils is viscosity modifiers (polymers) added to stabilize the viscosity. These modifiers are long-chain molecules that lessen the change in viscosity as the temperature changes. These additives are broken into smaller chains by high shear forces, i.e., between the bearings and cams or crank. Shearing of the oil occurs when its molecules are cut into smaller molecules.

Two processes cause this, 1) heat and pressure from the engine and 2) mechanical shearing as the molecules are physically cut by engine components. As a result of this mechanical wear, and the resulting broken molecular chains, the oil now changes viscosity much more rapidly as the temperature changes.

The oil can be too thin, especially at high temperatures, and the oil can be too thick, especially at low temperatures. The proper viscosity is one of the most important criteria of a lubricant. At low temperatures, the polymer chains are coiled up and allow the multigrade oil to flow just like the low number.

At -20 deg F, the 5W-20 oil flows like 5W oil. As the oil heats up, the polymers unwind into long chains, which prevents the oil from thinning as much as it normally would. At 212 deg F, the 5W-20 oil flows like 20-weight oil. In other words, a 5W-20 oil is a 5-weight oil that will not thin more than a 20-weight oil when it gets hot.

API Oil Service Ratings

In 1996, the American Petroleum Institute introduced the “SJ” service symbol for all automotive engines at that time. The status of “SJ” oil is now considered obsolete, as it was replaced in 1998 by “SL” grade oil. The API rating for the grade of oil used in used for a modern police engine, regardless of oil brand or viscosity, should have at least an “SL” rating.

The more modern, higher rated “SL” oil rating is correct for 2004 and older engines. The latest oils with the “SM” rating can be used in all automotive engines currently in use. Both “SL” and “SM” grades are considered current. Expect the “SL” grade to be phased out in the near future. The grade is marked on the label. Look for the term “API SERVICE SM.”

Engine type by engine type, GM performed oil analysis under different engine speeds and oil temperatures. It established a temperature-weighted, rpm based algorithm to calculate when each individual engine should have the oil changed. The most common limiting factor in petroleum-based (mineral) engine oil is oxidation stability.

But GM also factored in oil acidity, oil alkalinity and the changes in viscosity caused by wear. These four factors helped design the overall algorithm. From there, the engine speeds and oil temperatures experienced by each individual engine in each patrol car determine the oil change interval for that particular vehicle.

GM stands behind its engines maintained using the OLS. It saves money, saves labor, reduces waste oil and increases uptime. And all your officers have to do is watch for the dash light.

Published in Police Fleet Manager, Jul/Aug 2006

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