Clean Vs. Dirty Engine Oil – How to Differentiate?

There is a good reason why getting regular oil changes is extremely important. As oil ages and goes through numerous heat cycles, it not only degrades and stops lubricating the moving components efficiently, which is bad enough, but it gets dirty as well and collects all kinds of debris.

As for clean vs. dirty engine oil debate, the main differences include color, the amount of debris and other contaminant contents, viscosity, and slickness. In other words, dirty engine oil is darker, has a high content of contaminants, has a lower viscosity, and because it’s less slick, it doesn’t create a good enough protection film.

What Causes Engine Oil To Become Dirty

Dirty Engine Oil

There are a lot of dirty engine oil causes, all of which are natural unless there is something wrong with the engine. One of the main oil contaminants is metal shavings or engine wear particles.

These are tiny, almost microscopic metal pieces that are produced by metal components sliding against each other. Again, this is normal, but if you skip a few oil changes, these shavings can clog up the oil filter or end up in critical engine components resulting in increased wear or failure.

The second common contaminant is the fuel which is especially a problem if your car has issues with some components that enrich the air/fuel mixture. Fuel makes the oil too elastic and thin, which equals insufficient lubrication. That said, a small amount of fuel will always be present in old oil, but enough that you can smell it.

The next contaminant is what makes old oil darker in color, and that’s carbon deposits and soot. No matter what type of engine is in your car, some amount of carbon deposits will always accumulate inside. And since oil has detergents that clean those deposits, they will end up in the oil pan and turn the oil black or make it noticeably darker.

Also, moisture is a very common contaminant. Although it’s almost undetectable, a certain amount of moisture will enter the engine through the intake and end up in the oil pan. That makes the oil aerate and become too thin, again resulting in a decreased lubrication performance.

And lastly, we have sludge. All engine oils are prone to oxidization which is accelerated when the oil is at higher temperatures. That means older engine oil has spent more time at higher temperatures and is at a higher level of oxidization. And oxidized engine oil turns into sludge, making it too thick and difficult to flow.

Dirty Engine Oil Symptoms

Unless your engine oil is extremely dirty, which happens only if you skipped a few oil changes or there was something wrong with the engine, you likely won’t notice any dirty engine oil symptoms. That’s because marginally dirty oil doesn’t show any obvious symptoms until it’s too late, or it causes irreversible engine damage.

To avoid that, get your oil and oil filter changed at manufacturer-set intervals with the correct oil type. But if something is already wrong, here are some dirty oil symptoms you may notice, and keep in mind that dirty engine oil symptoms are largely the same as overdue oil change symptoms.

How To Keep Car Engine Oil Clean

Clean Engine Oil

There are no way you can keep engine oil clean than getting it changed regularly and making sure the car and all its components are in good working condition. That means to avoid driving with a faulty MAF sensor, a faulty O2 sensor, leaking injectors, vacuum leaks, dirty air filters, etc. In other words, anything that can affect enriches or leans out the air/fuel mixture.

A rich air-fuel mixture will increase carbon deposits that will end up in the oil, and a lean air-fuel mixture will increase the contents of other contaminants like moisture. And, of course, old engine oil turns into sludge which can wreak havoc on pretty much all internal components.

Also, if your engine has already had issues with dirty engine oil, you can try using high-mileage motor oil formulas like Mobil-1 High Mileage or Castrol High Mileage. Those formulas have extra detergents that will help clean out the engine. Plus, as a more extreme measure, you can try engine flush oil additives like Liqui Moly Engine Flush Plus.

Clean Vs. Dirty Engine Oil Difference

If we were to send brand new oil and dirty oil for chemical analyses, there would be a world of difference between clean vs. dirty engine oil. That means even though the difference isn’t so noticeable to the naked eye, it doesn’t mean it’s not there. And with that, here are all the differences, some of which you can test for yourself and others only by sending a sample for analysis.


As oil gets old and dirty, instead of gold, it becomes dark amber, brown, or even black. The reason it becomes amber or brown is that it burns due to all the heat cycles. And it turns black because of carbon deposits and soot that it picked up inside the cylinders and valvetrain.

On the other hand, brand-new and clean oil will always be golden in color and much more see-through than old and dirty oil. That is most noticeable when you check the oil dipstick right after an oil change, and you can clearly see the metal under the oil or the color of the dipstick tip.


Viscosity is difficult to notice with bear hands, especially considering that all modern oils have a different viscosity at higher temperatures. Still, with dirty and old oil, that viscosity will probably be thinner at both cold and operating temperatures. However, extremely old oil could even become gooey or thick. The viscosity of brand-new oil, on the other hand, is always true to what the manufacturer puts on the label.


The quality of the oil is difficult to tell without an analysis, but it’s not impossible. The best way to check the quality is to buy some brand-new oil and compare it to the oil you get on the dipstick. Usually, you will notice that dirty engine oil doesn’t leave a fine protection film between your fingertips, and you will soon start to feel your fingerprints rubbing against each other. Then, try the same thing with brand-new oil, and the difference will be easy to notice.


You won’t find any contaminants in brand-new oil, not even moisture. However, dirty oil, even with a healthy engine, will contain metal shavings, fuel, coolant, carbon, etc. And while that’s normal, the amount of those contaminants can vary. And while you can’t check the oil for moisture, you can for fuel, metal shavings, and carbon.

If the oil has too many carbon deposits, it will start turning black instead of brown or dark amber. Furthermore, if it contains fuel, it will be easy to smell it. And lastly, you can drain the old oil into a bucket and slush it around to see if you notice any sparkles. If you do, those sparkles are metal shavings.

Why Does Diesel Oil, Turn Black

The reason diesel engine oil turns black much more than it does in gasoline engines is direct fuel injection, higher compression, and the EGR (exhaust gas recirculation system). The higher compression increases blow-by, resulting in more carbon deposits in the oil pan.

The same goes for the direct fuel injection system, which sprays fuel directly into the cylinders making it harder to atomize and burn off, resulting in a richer air/fuel mixture for a brief time. That also facilitates more carbon deposits in the valve train, which ultimately ends up in the oil.

Lastly, the EGR system recirculates exhaust gases into the intake and, consequently, the cylinders. Given how exhaust gases are full of carbon and soot, it’s easy to see how that would further facilitate carbon deposits in the valve train, cylinders, and, ultimately, oil.


Q: Is dirty oil better than no oil?

Yes, dirty oil is better than no oil. As a matter of fact, any type of oil, be it conventional or synthetic, and any viscosity, be it 0W-30 or 20W-60, is better than no oil. All you need to know regarding this question is that an engine with no oil will run for about a minute before it’s ready for the scrap yard.

Q: What happens if you add clean oil to dirty oil?

When you add clean oil to dirty oil, they will mix together and reduce the number of contaminants per a specific amount of oil. Nothing bad will happen because of that, and essentially that’s what you are doing every time when you top up the oil.

Q: How does dirty engine oil look?

If the oil is dirty because of carbon deposits and soot, it will be black or extremely dark brown color instead of gold. Other contaminants don’t change the color of oil, but old oil will become dark amber or brown, but all you need to do is compare the color of your oil with new oil, and if the difference is too big, it’s time for an oil change.

Q: Does synthetic oil get dirty?

Yes, synthetic oil, just like conventional and synthetic blends, gets dirty with all the same contaminants. However, synthetic oil is much less prone to burning, oxidation, and performance issues caused by contaminants. Also, synthetic oil is much better at cleaning carbon deposits, so it can become even dirtier than conventional oil.

Q: What does clean oil look like on a dipstick?

Clean oil on a dipstick will look gold or amber in color, plus you will be able to see the dipstick through it. On the other hand, dirty oil is dark brown or black and hardly see-through.

Q: What color is good oil compared to bad oil?

Good oil is always golden in color and very transparent. You can find a lot of photos online to see what that looks like. Bad oil can range anywhere from dark amber to black, with little to no transparency.

Q: Does clean oil improve performance?

No, clean oil doesn’t noticeably improve performance. Clean oil does reduce friction between moving parts which increases engine power to a certain degree, but enough for it to be detectable unless the previous oil had over 20,000 miles on it.

Q: How can you tell if the oil is clean or dirty?

The best way to tell if the oil is clean or dirty is to send a sample of it for analysis where they can discover if and what is wrong with the engine. Other than that, the only way to tell if oil is dirty is by color. Dirty engine oil is dark amber, brown, dark brown, or black, while clean oil is golden or light amber.

Final Words

In the end, the main difference between dirty and clean engine oil lies where you can’t see it, and that’s lubrication performance. Other than that, the differences include viscosity, contaminant contents, color, and quality.

The viscosity and contaminants can only be detected by an oil analysis. However, a dark amber, brown, or black color is a dead giveaway that oil is dirty. Also, you can check the quality of the oil by sliding it between fingers while comparing it to fresh oil, but this method is highly subjective.

Ibro Cehic

Ever since I was bitten by the automotive bug during early childhood I was obsessed with cars. My first driving experience came when I was ten and I already started tinkering with cars and motorcycles at thirteen. So, right from the beginning, I knew my life would revolve around cars, even if I wasn’t sure how that would happen. And today, thanks to my second passion, writing, I get to share my love for automobiles with other enthusiasts through my articles.

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