How Does Motor Oil Become Contaminated?

Motor oil deteriorates and becomes unfit for service due to accumulation of contaminants in the oil and chemical changes (additive depletion and oxidation) in the oil itself.

How to change your oil


• Dust and Dirt
The design limitations of air cleaners, some oil fill caps and crankcase ventilation systems allow a certain level of dust and dirt into the engine, while leaks in the intake system can permit unfiltered air to enter the engine. Proper maintenance of the engine and its accessories can minimize the amount of contaminants entering the lubrication system and extend engine life.

• Metal Particles
Normal wear of engine parts produces very small metal particles that are picked up and circulated by the oil. Particles of road dust and dirt increase wear rates and generate larger, even more abrasive metal particles that are circulated through the engine by the oil. While oil filters help keep these particles at a minimum, they can’t remove them entirely.

Combustion By-Products

• Water
Combustion produces water vapor, or steam. When engine temperatures are high, most of the water remains in vapor form and exits through the exhaust. However, when engine temperatures are low, such as at start-up, warm-up and during short-trip operation in low ambient temperatures, the water vapor condenses (turns into a liquid) on cylinder walls and enters the crankcase oil, where it leads to sludge and corrosion.

• Acids
The combustion process produces acidic gases which, like water vapor, condense on cylinder walls at cold engine temperatures and find their way into the crankcase oil. The gases combine with water to cause corrosion.

• Soot and Carbon
Incomplete combustion produces soot, carbon and other deposit-forming materials. An engine running too “rich,” or with too much fuel, increases contaminant levels. Light-load, low-speed gasoline engine operation and high-load, low-speed diesel engine operation increase levels of these combustion by-products.

• Dilution
When an engine is started or running abnormally, some unburned fuel is deposited on cylinder walls, leaking past the rings and into the crankcase, where it reduces oil viscosity. Fuel dilution decreases oil film strength and increases oil consumption. Although this is only a minor problem when engine operation is at high-speed or high-temperatures, it can be a significant problem in vehicles consistently used for short-trips.

Although all motor oils eventually succumb to contamination and must be changed, high-quality synthetic oils – like AMSOIL synthetic motor oils – deliver superior protection for longer due to their higher-quality base stocks and additives.


  1. I’m a 74 year old woman who needs help finding a place that will do a test on old oil. I paid a so called friend and mechanic to change my oil. With the money I gave him for oil and filter he bought crack. He put used oil from the salvage yard in my engine. I had a $769 repair bill a valve went out. Now I have 3 injectors out. I want to get the oil tested he put in thanks

  2. E 350 MB: V-6
    I recently learned that my local Mercedes Benz dealer has been/does change oil in customer
    vehicles by suction tube through the dipstick tube. Apparently, this is their policy.
    I disagree with this method …period.
    Yesterday, I paid $180 to change my synthetic oil and filter a second time from the drain plug because a week ago the
    dipstick showed black oil upon driving it 3 miles home on prior change. Embarassed to pay that much and
    get inferior job for past oil changes too.
    I surmise that repeated suction type oil changes just allow contaminants to concentrate
    over time…putting clean oil into remnant in dirty pan.
    What are your comments? Advice?

    I am changing vendors for the future….

    1. Hi,

      If I were you, I’d visit an independent mechanic for oil changes. Thanks to the Magnuson-Moss Warranty Act, the OEM can’t force you to visit the dealer to maintain your factory warranty. Check out this site for details.

      Thanks for reading.

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