Why is There Dirty Oil on the Dipstick? (And What Causes It)

It’s a good idea to check your oil about every time you fill up the fuel tank. But what do you do with that information? What does dirty oil on the dipstick look like, and when is it time for an oil change?

You can’t always tell how old your oil is just by looking at it. Here are some signs that your oil still has some life left in it, and when you need to change it.

How Should Oil Look on a Dipstick?

Engine oil on a dipstick should be light to medium brown in appearance with no visible contaminants. Hold the dipstick up to the light and you should be able to see through the oil, at least a little bit.

How Does Engine Oil Get Dirty?

As you drive your car, the oil in your engine is exposed to oxygen during combustion. This exposure to oxygen is called oxidation, and it’s the same process that forms rust on metal over time.

Oxidation is the primary cause of engine oil breakdown. When engine oil oxidizes, it increases in viscosity (gets thicker). Debris and engine sludge are more likely to form in small passages as time goes on. The extreme heat created by the combustion process accelerates oxidation.

Additionally, the engine oil and filter pick up any dirt or debris that makes its way past the engine air filter. The oil filter provides a second line of defense for this debris to keep contaminants out of the combustion chamber.

Carbon from the combustion process also makes its way into the oil. This carbon is what turns the oil black very quickly after changing the oil.

Clean vs Dirty Engine Oil

The following image illustrates the difference between brand new oil and oil that has 10,000 miles on it.

clean vs dirty engine oil

What Does Clean Oil Look Like?

Fresh oil is amber in color, almost like honey or a very light maple syrup. As you drive, hydrocarbons in your engine are picked up by the oil, which turns the oil a light brown color. This process can easily happen in less than 100 miles.

Obviously your fresh oil is good for much longer than 100 miles. If you can still see clearly through the oil, even if the oil is brown, this is a good sign that the oil is fairly fresh.

Synthetic oils can go much further than conventional oils before the next oil change. This is because the components in synthetic oils are manufactured to be approximately the same size, and break down much more slowly than conventional oil. Synthetic oils are manufactured in a lap, and conventional oils are derived from crude oil.

What Does Dirty Oil Look Like?

Dirty engine oil is typically opaque and very dark brown or nearly black. You can’t see through dirty oil much, if at all. If the oil is very dirty, you may notice small particles or debris in the oil on the dipstick.

If your engine is old or is wearing prematurely, you might notice fine glitter, metal flakes, or rust on the dipstick as well. Metal on the dipstick can be cause for concern. To investigate further, you may want to open up a used engine oil filter or even drop the oil pan.

When Should I Change My Oil?

Your oil change interval will depend on your vehicle’s engine, the type of engine oil you’re using, and how you drive. Check your owner’s manual for a good starting point with your oil change interval.

As a rule of thumb, conventional oil should be changed every 3,000 miles and full synthetic oil should be changed every 6,000 miles.

If you drive the vehicle at high RPM, tow a heavy load, or make frequent stops (like you would in city traffic), you typically want to reduce this interval even further. In most cases, full synthetic oil should be changed every 3,000 miles in these severe driving conditions.

1500 miles synthetic mazda miata
This full synthetic oil has 1,500 miles on it, but is about a year old. It is generally recommended to change your oil every year, even if you don’t hit the mileage.
4000 miles synthetic oil subaru wrx sti
This full synthetic oil has around 4,000 miles on it. According to a used oil analysis, this oil has plenty of life left.

Some full synthetic oils are designed for extended use, and advertise oil change intervals up to 20,000 miles. If you choose to run an extended oil change interval, check with the manufacturer of that specific engine oil and consider performing a series of used oil analyses with the oil that comes out of that engine.

You’ll also want to continue to check your oil about every time you get gas to make sure you don’t have excessive oil consumption.

Analyzing Oil Quality

oil analysis kit
Engine oil analysis kit (Credit)

The best way to tell when you need to change your oil is to pay for a used oil analysis (UOA). There are several companies that will take an oil sample you’ve collected from your engine and break it down into all of its individual components, along with the parts per million (PPM) of each element found in the oil.

Blackstone Laboratories is a popular company that tests oil and fuel for elements in the oil, oil viscosity, insoluble material in the oil, and the oil’s flashpoint. The best way to perform used oil analyses is to send in an oil sample each time you change your oil. This allows the UOA company to build a trend of your engine.

For instance, if the lab sees a sudden increase in copper (an element commonly found in rod bearings), you might need an engine rebuild in the near future. If there is coolant in the oil, you may have a head gasket leak.

What Oil Should I Use?

Which oil you should pick is a matter of great debate. Some people swear by a different viscosity than the one recommended by the manufacturer.

Stick with the manufacturer’s recommended oil viscosity unless you have a really good reason for deviating from their recommendation. Modern engines have tighter clearances and can run 5W-30 or even 0W-20 safely if the engineers designed the engine around that particular specification.

No matter which oil you choose, check the owner’s manual for the API certification. Make sure the oil you pick meets or exceeds those specifications. Diesel oil such as Shell Rotella T6 contains different additives than you would find in engine designed for gasoline engines.


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