Engine oil is subjected to heat, oxygen, combustion gases, fuel, moisture and mechanical stress every time an engine operates. At the same time, its additives are continually working to control deposits, prevent wear, neutralise contaminants and protect internal engine components.
Eventually, the combination of chemical change, contamination and additive depletion reaches a point where the oil is no longer suitable for continued service. That is why even the highest-quality synthetic engine oil eventually needs to be changed.
Engine oil deteriorates because it is continually exposed to heat, oxygen, combustion by-products, moisture, fuel and mechanical stress. At the same time, the additives that protect the engine are progressively consumed or depleted. An oil change removes the degraded lubricant and accumulated contamination and replaces it with a fresh supply of oil and additives.
Engine Oil Has a Difficult Job
Engine oil does considerably more than provide a slippery film between moving parts. It must control friction and wear, carry heat away from components, suspend contaminants, control deposits, resist corrosion and continue flowing correctly across a wide range of temperatures.
Modern engines make these requirements particularly demanding. Turbocharging, direct fuel injection, high specific power output, emissions-control systems and extended service intervals can all place significant stress on the lubricant.
A modern engine oil is therefore a carefully balanced combination of base oils and additives. Both are affected by continued service.
1. Oxidation Changes the Oil
Oxidation is one of the fundamental processes involved in lubricant ageing. It occurs when the oil chemically reacts with oxygen.
Oxidation can cause the lubricant to thicken and contribute to the formation of acids, varnish and deposits. As oxidation progresses, the physical and chemical properties of the oil move further away from those of the fresh lubricant.
Engine oils contain antioxidant additives designed to slow this process, but antioxidants have a finite service life. Eventually they can become depleted and oxidation can accelerate.
2. Heat Accelerates Oil Deterioration
Heat and oxidation are closely related. Higher lubricant temperatures generally increase the rate of chemical reactions, accelerating oxidation and placing greater demands on the oil's antioxidant system.
Some areas of an engine can expose oil to particularly high temperatures. Turbocharger bearings, piston-ring areas and other highly loaded components can subject small quantities of lubricant to considerably greater thermal stress than the average oil temperature in the sump.
A quality synthetic base oil generally offers excellent resistance to high-temperature oxidation, which is one reason synthetic lubricants are particularly useful in demanding modern engines.
3. Fuel Can Dilute Engine Oil
A small amount of unburned fuel can reach the crankcase during normal engine operation. Cold starts and short trips can increase this effect because fuel may not completely evaporate from the oil before the engine is switched off again.
Direct-injected engines and engines experiencing particular operating or mechanical conditions can also be susceptible to fuel dilution.
Unlike oxidation, which can cause oil to become thicker, fuel dilution can reduce viscosity. Excessive dilution can weaken the lubricating film separating loaded components and compromise wear protection.
A noticeable increase in sump oil level or engine oil with a strong fuel smell can indicate excessive fuel dilution. This may require diagnosis rather than simply replacing the oil.
4. Moisture Accumulates in the Crankcase
Water is another contaminant engine oil must manage. Moisture can form through condensation as the engine repeatedly heats and cools, while combustion also produces water as a by-product.
When an engine reaches full operating temperature and remains there, moisture can evaporate and leave through the crankcase ventilation system. Repeated short trips may not provide enough time for this to occur effectively.
This is one reason a vehicle that travels relatively few kilometres can still require regular oil changes. Engine-oil life is influenced by operating conditions as well as distance travelled.
5. Combustion By-Products Contaminate the Oil
The piston rings cannot create a perfect seal between the combustion chamber and crankcase. A small amount of combustion gas passes the rings during operation, a process known as blow-by.
These gases can introduce soot, partially burned fuel and other combustion by-products into the engine oil. Detergent and dispersant additives help control this contamination and keep material suspended until it can be removed during an oil change.
As contamination accumulates, however, the lubricant and its additive system have progressively more material to manage.
6. Dirt and Wear Particles Enter the Oil
An engine's air and oil filtration systems are designed to control solid contaminants, but no system can eliminate every particle under every operating condition.
Dust can enter through the intake system, particularly if an air filter or intake connection is damaged. Normal engine wear can also produce microscopic metallic particles.
The oil filter removes contaminants within the range it is designed to capture, while smaller particles can remain suspended in the lubricant until the oil is drained.
7. Mechanical Shear Can Change Viscosity
Engine oil is repeatedly subjected to intense mechanical forces as it passes through bearings, oil pumps, valvetrain components and other highly loaded areas.
Multigrade lubricants can use viscosity-modifying polymers to help achieve the required viscosity characteristics across a broad temperature range. Depending on the formulation, these molecules can be subjected to mechanical shear during service.
Permanent shear can reduce the lubricant's high-temperature viscosity. High-quality oils are formulated to resist unacceptable viscosity loss throughout their intended service interval.
8. Engine-Oil Additives Don't Last Forever
The additives in engine oil are not simply inactive ingredients waiting in reserve. Many perform their function by reacting chemically, neutralising contaminants or interacting with engine surfaces.
A modern additive package can contain several types of chemistry, including:
- Detergents to help control deposits and neutralise acidic contaminants.
- Dispersants to help keep contamination suspended in the oil.
- Anti-wear additives to protect highly loaded surfaces.
- Antioxidants to slow lubricant oxidation.
- Corrosion inhibitors to help protect internal metal surfaces.
- Friction modifiers where required to influence frictional characteristics.
- Anti-foam additives to control foam and entrained air.
- Viscosity modifiers where required to help the oil provide multigrade performance.
As the oil remains in service, some additives are progressively consumed, chemically altered or depleted. Eventually the lubricant no longer has the same ability to protect the engine that it had when new.
An oil change isn't simply about removing dirty-looking lubricant. Used engine oil can contain accumulated contaminants, depleted additives and chemically altered base oil even when its appearance alone doesn't suggest a problem.
Does Engine Oil Always Become Thicker as It Ages?
No. Different deterioration mechanisms can push viscosity in opposite directions.
Oxidation, soot and some forms of contamination can increase viscosity. Fuel dilution and mechanical shear can reduce it.
Several processes can also occur simultaneously. An oil's final viscosity therefore depends on the engine, lubricant, operating conditions and contaminants present.
This is one reason laboratory used-oil analysis can provide considerably more information about lubricant condition than appearance alone.
Why Does Engine Oil Turn Black?
Dark engine oil does not automatically mean the lubricant has reached the end of its service life.
One function of detergents and dispersants is to control contamination and keep deposits from accumulating on engine surfaces. As contaminants become suspended in the lubricant, the oil can darken considerably while continuing to perform correctly.
Conversely, oil that still looks relatively clean is not necessarily fit for continued service. Oxidation, fuel dilution and additive depletion cannot be reliably judged simply by looking at the dipstick.
Why Synthetic Oil Generally Resists Deterioration Better
Synthetic lubricants can be engineered using base oils with properties particularly suited to demanding engine applications. High-quality synthetic base oils can provide excellent resistance to oxidation, low volatility and strong performance across a broad temperature range.
This can reduce some of the limitations associated with less highly refined base oils, particularly under extreme temperature conditions.
However, the word synthetic by itself does not determine the quality or service life of an engine oil. The finished lubricant's base-oil system, additive package, viscosity and performance specifications all matter.
Synthetic oil also remains subject to contamination and additive depletion. It may resist deterioration extremely well, but it does not last forever.
What Happens If Engine Oil Is Left in Service Too Long?
Eventually, deterioration and contamination can compromise the oil's ability to perform one or more of its required functions.
Depending on the conditions involved, the result can include increased deposits, sludge formation, viscosity change, corrosion, reduced wear protection and restricted oil circulation.
Severe sludge can restrict oil galleries and the oil-pump pickup, potentially starving critical components of lubricant. For a detailed explanation, see our guide to what causes engine sludge.
How Do You Know When Engine Oil Is Worn Out?
For normal road vehicles, the practical answer is to follow an appropriate service interval rather than trying to judge the condition of the oil visually.
The vehicle manufacturer, oil-life monitoring system and lubricant manufacturer can all provide relevant service recommendations depending on the application and oil being used.
For specialised equipment, competition engines, heavily modified vehicles or applications where optimising drain intervals is important, laboratory used-oil analysis can measure characteristics such as viscosity, contamination, wear metals and other indicators of lubricant and engine condition.
For more information about selecting an appropriate service interval, see How Often Should You Change Synthetic Engine Oil?
AMSOIL Synthetic Engine Oils
AMSOIL synthetic engine oils are formulated using synthetic base oils and advanced additive systems designed to resist oxidation, control deposits and maintain protection throughout their intended service life.
Different AMSOIL engine-oil families are designed for different applications and service requirements. Some are designed around manufacturer-recommended service intervals, while others are formulated for qualifying extended-drain applications.
The correct service interval therefore depends on the specific AMSOIL product, vehicle and operating conditions rather than simply the fact that the lubricant is synthetic.
Engine oil doesn't wear out for one single reason. Heat and oxygen chemically alter it, contaminants accumulate, mechanical forces can affect viscosity and additives are progressively consumed. High-quality synthetic oil can resist many of these processes exceptionally well, but every engine oil eventually reaches the end of its useful service life.
Not sure which engine oil your vehicle requires? Use the AMSOIL Vehicle Finder, or visit our Engine Oil Standards Guide to understand the specifications required by your engine.
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