What causes the engine oil to get dirty?

This blog post will answer the question, ‘What causes the engine oil to get dirty?’, and cover topics like how to know if the engine oil is dirty, causes behind the engine oil getting dirty, and reasons behind using engine oil additives.

What causes the engine oil to get dirty?

These are the following causes that make the engine oil dirty:

  • An overheated engine
  • An increased rate of oxidation
  • Clogging of the oil filter
  • A faulty PCV valve(Positive Crankcase Ventilation)
  • Engine wear and tear
  • Buildup of dirt
  • Diluted fuels
  • Contamination of water
  • Leakage of the coolant

How to know if the engine oil is dirty?

Dirty engine oil can be spotted by noticing the color of the oil. When the engine oil is brand new, it appears in a translucent and amber color. Engine oil gets dirty over time and frequent usage and can be identified by noticing a thick and dark oil.

If the oil color changes to a dark brown or a grey shade and possesses a foamy texture, it is indicative of the contamination of the engine oil due to a leakage in the coolant.

In case of the engine oil appears dark red, it means that there has been a leakage of fluids present in an automatic transmission. Contaminated engine oil becomes thicker and loses its viscosity and fails to lubricate the engine components like it used to.

In some cases, running a car engine on contaminated engine oil for a long can result in a knocking sound from under the hood. This is because a lack of oil in and around the moving parts limits their lubrication and thus causes metal parts to hit against each other.

Causes behind the engine oil getting dirty

These are the various causes contributing to an engine oil getting dirty.

An overheated engine

The optimum operating temperature of an engine is somewhere between 200-240 degrees. If the engine undergoes severe and extensive overheating, the engine oil can be affected.

The reason behind the engine oil getting affected is that at higher temperatures, the heat interferes with the airflow inside the engine and attacks the engine oil viscosity.

The viscosity of an oil is what shields it from high temperature and pressure conditions. Excessive overheating can break this shield and interfere with the oil molecules thus breaking them apart. This causes makes the oil vulnerable to external factors such as dirt, dust, debris, etc. and in turn, makes it dirty.

Increased rate of oxidation

Oxidation affects the engine oil by causing it to break down over time. Oxidation of the engine oil leads to a reduced effect of lubrication for the various engine components. 

As the oil thickens and is subjected to further oxidation, the development of sludge takes place, and sludge being thick and gooey in texture blocks the oil pathways and thus shuts off most of the lubrication for the engine components.

Clogging of the oil filter

The oil filter is used to eliminate all the dust, dirt, and grime buildup accumulated in the oil during its course inside the engine. As time passes by, the oil filter gets clogged and filled with these contaminated materials.

Running a clogged oil filter for long can also make the oil dirty as it has to pass through the filter for further access to the components. Changing the oil filter becomes an absolute necessity during every oil change.

Faulty positive crankcase ventilation(PCV)

The positive crankcase ventilation system(PCV) is responsible for controlling the flow of the exhaust gases from the engine and recirculating them back to the combustion chamber for further combustion.

In case the positive crankcase ventilation system gets clogged due to the buildup of dirt, dust, gunk, etc. it can no longer control these exhaust or blow-by gases.

This causes the blow-by gases to find a way into the oil sump that holds the engine oil. On contact, they start contaminating it with the residues from the combustion process and cause it to not function properly.

Engine wear and tear

This usually takes place in cold weather when the engine cannot start and needs to be revved up to get the oil flowing through the engine components and start up the car.

While performing this activity, small chips of engine metal get scraped off and enter the engine oil. These fine metal bits can easily get contaminated and make the oil dirty. To avoid this it is recommended to use a winter grade oil inside the engine to start the car up faster.

Leakage of coolant or water into the engine oil

These substances cannot directly affect the engine oil, but should not be mixed with the engine oil. It affects the engine oil by raising its viscous nature, which acts as a hindrance to its smooth flow to the other engine components.

Reasons behind using engine oil additives

These are the following reasons why engine oil additives should be used.

Better lubrication

A better lubricating capability of the engine oil can help it to eliminate any dirt, dust, or gunk buildup inside the engine oil easily as it is able to move freely.

Reduction in heating

Using oil additives in the engine oil can help to protect it from any wear and tear caused due to high temperatures and thus help in reducing the friction faced.

Maintaining oil viscosity

Oil additives also help in retaining the original viscosity of the oil by reducing its breakdown and delivering optimum performance irrespective of the weather conditions.

Research has shown the merits and demerits of using oil additives in car engine oil.


This blog post addressed the question, ‘What causes the engine oil to get dirty?’

We understood the ways to tell if the engine oil is dirty and came across the different causes behind the engine oil getting dirty. Furthermore, the reasons behind using oil additives for engine oil have also been discussed. Please feel free to comment on the content or ask any questions in the comments section below.

Frequently Asked Questions (FAQs): What causes the engine oil to get dirty?

Why does car engine oil get dirty?

It is common for a car engine oil to get dirty after a certain amount of time as it receives bits of ash from the combustion cylinders and also metal chips and carbon deposits from the engine startups.

How do you know if your engine oil is bad?

The engine oil is said to be bad if the car suffers from a sudden increase in mileage, distinct knocking sounds coming from under the hood, or if the oil is found to be dark or dirty.

How long can you drive on dirty oil?

Generally, cars are able to run up to 6000-7000 miles on dirty engine oil and even more if the car uses synthetic oil.