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Why Does Motor Oil Turn Black?

Written by Charleen

Motor Oil Turning Black Isn’t an Indicator of Bad Oil

John Baker|

What causes black motor oil? And when your oil darkens does it mean it’s time to change it? Well, there are a couple of factors that can cause the former. Let’s dig in.

Factors causing black motor oil

Heat cycles naturally darken motor oil

During your drive to work in the morning, your engine reaches normal operating temperature (typically 195ºF-220ºF), heating the motor oil. Then the oil cools while your car sits in the parking lot. During lunch, the oil again is exposed to heat during your drive to Walmart for butter and shoe laces. The process repeats on the way home. And the next day. And the next.

That’s what’s meant by “heat cycles.” The continual exposure to periods of high heat naturally darkens motor oil.

Some additives in motor oil are more susceptible to darkening in the presence of heat than others. In addition, normal oxidation can darken oil, too. Oxidation occurs when oxygen molecules interact with oil molecules and cause chemical breakdown, just like how oxygen causes a cut apple to brown or iron to rust. High heat accelerates oxidation.

Soot causes oil to turn black

While heat cycles cause oil to darken, soot causes oil to turn black. Most people associate soot with diesel engines, but gasoline engines can produce soot as well, particularly modern gasoline-direct-injection engines.

Soot is a byproduct of incomplete combustion. Since soot particles are less than one micron in size, they typically don’t cause engine wear. For comparison, a human hair is roughly 70 microns in diameter.


If soot particles agglomerate into larger wear-causing contaminants, the oil filter will catch them. Sometimes people who use bypass filtration systems, which can filter contaminants down to two microns, express surprise that the motor oil is still black. Soot, however, can still elude filtration down to two microns. Any finer filtration and the filter could catch dissolved additives in the motor oil.

Oil Myth: The color of the oil indicates when it’s time for an oil change

It’s common to assume that black motor oil has worn out or become too saturated with contaminants to protect your engine and requires changing. Not necessarily. As we saw, discoloration is a natural byproduct of heat and soot particles, which are too small to wear out your engine.

The only surefire way to determine if the oil has reached the end of its service life is to perform oil analysis. Chemically analyzing an oil sample reveals the condition of the oil, the presence of contaminants, fuel dilution and so on. Several companies offer oil analysis services, including Oil Analyzers INC.  We keep the kits here in Sioux Falls

Absent oil analysis, it’s best to follow the oil-change recommendation given in your vehicle owner’s manual or by the motor oil manufacturer. The recommended service intervals for AMSOIL products, for example, are based on thousands of data points spanning years of use.

It’s best to trust the data, not your eye, in this case. Otherwise, changing the oil could amount to throwing away good oil.

Time for an oil change? Find AMSOIL product for your vehicle here.

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Signature Series Battles Sludge

Written by Charleen

Signature Series Battles Sludge

The health of your engine depends on motor oil circulating quickly and efficiently through the system, but sticking components and obstructed passages inhibit motor oil from lubricating, cooling and protecting your engine. Engine “sludge” occurs when oxidized oil and contaminants build up on engine surfaces. It can restrict the flow of oil to the point of engine failure and costly repairs.

Warehouse note: The so called “fully synthetics” which are mostly group III’s or a combination of a majority group II and a fraction III ($3.50 synthetics)  to a majority III and a fraction of IV ($8.50 synthetics) – all based on price and what the stock holders allow to be used for a desired profit.. Note that all of these WILL leave deposits in the form of sludge. Turbocharged engines must avoid these brands which include the silver one shaped like a shampoo bottle. 

Sludge: Where It Starts & How It Ends

  • The valve cover and oil pan are generally the first areas sludge appears.
  • The oil pick-up tube screen is often the next spot it accumulates, impeding oil flow through the system.
  • What begins as a thin film of lacquer or varnish deposits eventually bakes into an expensive mess.

 

The Sequence VG Engine Test

Engine failures due to sludge are often caused by a clogged pick-up tube screen – the motor is effectively starved of oil. The Sequence VG Engine Test determines how well an oil resists sludge formation and keeps the lubricant flowing freely throughout the system. The test is required for API SN PLUS – a specification recommended by most domestic vehicle manufacturers.

TEST PARAMETERS

 

Engine Ford* 4.6L V-8
Duration 216 hours
Measures Sludge and varnish deposits, piston ring sticking, clogged oil pump screens and roller pin wear
Simulates Taxi, delivery or commuter vehicle service
Requirement Oil pick-up tube screen limited to 10 percent blockage

 

The Results

Signature Series Synthetic Motor Oil was subjected to the Sequence VG to measure its ability to prevent sludge. As expected, Signature Series produced an oil pick-up tube screen virtually free from sludge (see image). Our unique combination of detergents and high-quality base oils control oxidation and sludge to keep engines clean and efficient.

 

Signature Series has 50 percent more detergents1 to help keep oil passages clean and promote oil circulation. It provides 90% better protection against sludge2.

Warehouse Note: Thankfully our efforts here in Sioux Falls have made the Signature Series our top seller and on the minds of many who want their investment to remain like new beyond 250,000 miles. People are realizing that it’s not all about change frequency as a non “true synthetic” can cause damage in just 10 miles in GDI engines due to LSPI.. Not to mention why I adopted AMSOIL back in the day – the added power and performance!!

1 vs. AMSOIL OE Motor Oil
2 Based on independent testing of AMSOIL Signature Series 5W-30 in the ASTM D6593 engine test for oil screen plugging as required for the API SN PLUS specification.
*All trademarked names and images are the property of their respective owners and may be registered marks in some countries. No affiliation or endorsement claim, express or implied, is made by their use. All products advertised here are developed by AMSOIL for use in the applications shown.

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