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GM Oil Life System

On 2004 and newer General Motors 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 beyond 6,000 miles. This 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 GM Oil Life System may extend petroleum-based oils into the range of much higher-priced synthetic oils.

RPMs and Oil Temp

The OLS does not sense the condition of the oil, either the viscosity or acid content, is not keyed to time, and is not based on miles. Instead, 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 3500 rpm will need the oil changed sooner than an engine operated at 2000 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 engine revs are multiplied by the oil temperature penalty factor. Then this weighted or corrected number is subtracted from the maximum allowed engine revs. Note that this is oil temperature and not water/coolant temperature.

The GM Oil Life System is based on the kind of engines used in law enforcement and each engine has a logarithm developed specifically for that engine. 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.

During use, engine oil changes chemically due to 1) oxidation and 2) contamination with fuel, water, ethylene glycol, worn metal, and the soot byproducts of combustion. The additives like anti-oxidants, anti-wear, corrosion inhibitors, detergents, dispersants, and viscosity modifiers improve the longterm stability of the oil.

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.

Acids and Bases

Also part of the OLS logarithm 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. 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.


Viscosity is the oil’s resistance to flow with respect to temperature. The Viscosity Index is a measure of oil resistance to flow as the oil gets very hot or very cold. The Viscosity Index is increased (the oil acts thicker) due to oxidation, foaming or mixed 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) due to fuel or solvent decontamination, molecular shearing, or contamination by refrigerant.

The key to multi-weight oils are 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. 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 the most important criteria of a lubricant.

GM stands behind their 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.

Earl Gautsche is the National Police Fleet Service Manager for GM fleet. He can be reached at (248) 789-4679.

Published in Police Fleet Manager, Sep/Oct 2005

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