Engine oil plays a critical role in keeping your car’s engine running smoothly. As oil ages and deteriorates with use, its viscosity (or thickness) changes. But does it get thicker or thinner over time? Here’s a detailed look at how and why engine oil viscosity changes.
What Causes Engine Oil to Change Viscosity?
There are two main factors that cause engine oil to change viscosity as it ages:
- Oxidation – Exposure to high temperatures causes the oil’s molecules to break down, reducing viscosity.
- Contamination – Accumulation of particles and fuel dilution thickens the oil over time.
During normal engine operation, motor oil is subjected to high temperatures and pressures inside the engine. This leads to oxidation – breaking down of the oil’s molecular structure. As the large hydrocarbon molecules in fresh oil break into smaller fragments, the oil becomes thinner and less viscous.
At the same time, oil picks up contaminants and fuel residues, which increases its viscosity and thickness. Tiny metal particles from engine wear, dust and dirt from air intake, soot from combustion, and fuel residues like gasoline or diesel that seep past piston rings all get suspended in the oil over time.
The combination of oxidation and contamination have competing effects on oil viscosity. Whether the aged oil turns thicker or thinner depends on which process dominates.
Does Oxidation or Contamination Dominate?
In most modern engines under normal driving conditions, oxidation has a stronger effect than contamination. This causes the oil to get thinner as it ages.
Key reasons why oxidation dominates include:
- Better air filters prevent contaminants from entering oil
- Tighter piston rings prevent fuel from leaking into oil
- High engine temperatures promote molecular breakdown
However, in certain scenarios, oil contamination can exceed oxidation. This typically happens when:
- Engine has worn piston rings or cylinder walls
- Air filter is clogged or damaged allowing more dust in
- Vehicle operates under heavy load, producing more soot
- Oil change intervals are overly extended
Under such conditions, the build-up of particles, soot and fuel dilution can make the oil thicker over time.
Measuring Viscosity Changes
Oil viscosity is measured by a parameter called kinematic viscosity, in units of centistokes (cSt). Fresh oil has a viscosity rating as defined by SAE J300 standard.
For example, a 5W-30 oil has a kinematic viscosity of 9.3-12.5 cSt when new. As the oil degrades during use, its viscosity can reduce to 7 cSt or lower indicating significant oxidation.
On the other hand, badly contaminated oil can have viscosity exceeding 15 cSt. These viscosity changes directly impact the oil’s lubrication performance.
Effects of Viscosity Change
As oil viscosity changes with use, it affects lubrication performance in the following ways:
Thinner Oil
- Increased engine wear – Thinner oil provides less protection
- Oil consumption rises – Thinner oil leaks past piston rings easier
- Reduced fuel economy – Increased friction from lower viscosity
Thicker Oil
- Poor cold flow – Harder time circulating when engine is started
- Increased engine deposits – Thicker oil doesn’t flow as easily
- Higher pumping losses – More resistance to oil pump
Either way, badly degraded oil can accelerate engine wear and reduce fuel efficiency. That’s why regular oil changes are vital.
Conclusion
While both oxidation and contamination alter oil viscosity as it ages, oxidation has a stronger effect in most engines. This causes the oil to steadily thin out with use. Only under abnormal conditions does contamination make the oil thicker over time.
Viscosity changes either way impact the oil’s ability to protect and lubricate the engine. This demonstrates the importance of periodic oil changes before the viscosity strays too far from optimal levels.
So the takeaway is that under normal driving conditions, engine oil typically gets thinner as it ages rather than thicker. This thinning is accelerated by high engine temperatures and stresses. Allowing oil viscosity to reduce excessively can greatly shorten engine life. That’s why we have to change our oil regularly!
| Factor | Effect on Viscosity |
|---|---|
| Oxidation | Decreases viscosity (thins oil) |
| Contamination | Increases viscosity (thickens oil) |
Typical Causes of Excess Oil Contamination
- Worn piston rings or cylinder walls
- Clogged or damaged air filter
- Extended oil change interval
- Frequent heavy engine loads
Key Impacts of Changed Oil Viscosity
Thinner Oil
- Increased engine wear
- Higher oil consumption
- Reduced fuel economy
Thicker Oil
- Poor cold flow properties
- More engine deposits
- Higher pumping losses
Oil viscosity is measured in centistokes (cSt):
- New oil: 9.3-12.5 cSt
- Oxidized oil: Around 7 cSt
- Contaminated oil: Over 15 cSt
Regular oil changes are vital to maintain proper viscosity and ensure long engine life. Follow your vehicle’s oil change frequency recommendations.
In summary:
- Engine oil normally thins out as it oxidizes with use
- Contaminants can make it thicker if they build up abnormally
- Viscosity changes affect lubrication and engine protection
- Check oil viscosity as you service your vehicle
- Stick to the recommended oil change schedule