Black motor oil — What that means
Motor Oil Color is NOT an indication of Condition
Read this again: Motor Oil Color IS NOT AN INDICATION OF ITS CONDITION —
I’m amazed at the amount of misinformation propagated on the internet about how motor oil color somehow indicates its condition. Some sites even show bogus motor oil color charts that imply that color somehow indicates motor oil condition as it turns from a honey color to dark brown. Black motor oil can test perfectly fine in lab tests, but these color charts would have you believe that it must be changed.
This motor oil color chart is complete bullshit. There are absolutely no scientific studies to back up the premise that oil color is an indication of oil condition
There are no scientific studies that link motor oil color and condition
Everything you read about motor oil color somehow indicating motor oil condition is a myth and that myth started back in the early 1960’s when oil companies first developed detergent oils.
Detergent oil do what the name implies, they help clean your engine. But more importantly, after cleaning, detergent oil contains dispersant additives that keep the soot from blowby gasses in suspension. Suspension is important for two reasons; 1) it prevents the particulate matter from falling to the bottom of the pan where it can form sludge and 2) it carries the particulate matter to the oil filter so it can be removed.
Untrained shops started the myth that motor oil color is an indication of condition, and they keep it going to this day
Before detergent oil, the particulate matter in oil settled to the bottom of the pan. Once detergent oils came out, motor oil started looking dark. Why? Because it was doing its job; cleaning and carrying soot and particulate matter to the oil filter.
Untrained shops saw the dark oil and immediately claimed that the dark oil was a sign of an overdue oil change. They had never seen such “dirty oil” before.
What does black motor oil indicate?
All engines, even brand new ones, produce blow-by gasses; fuel, soot, and water that escapes past the piston rings and into the crankcase. Most of the blow-by is pulled out of the crankcase by the positive crankcase ventilation (PCV) system and routed back into the intake manifold where it is burned. But the rest of the blowby winds up in the oil.
As mentioned above, blowby contains soot, exhaust gasses, raw fuel, and water. It’s the oil’s job to clean this from the engine, hold it in suspension, and transport it to the filter to be removed. All do this all the time.
So it’s not uncommon to see your honey-colored one-week-old motor oil start to turn dark. It’s doing its job.
What actually turns motor oil black?
I’ve written an -in-depth article on this subject. Read it here. But the short version is this:
Why motor oil turns black
Soot from incomplete combustion turns oil black
Soot is most commonly present after a cold start and occurs more often in later-model engines with gasoline direct injection. The soot enters the crankcase as blowby
Oxidation
Oil and oil additives oxidize in the presence of heat and oxygen and can turn dark
Oil oil turns new oil dark
No matter how long you drain your oil during an oil change, there’s still oil black oil throughout your engine. That oil can immediately darken the new oil.
Which engines turn the oil dark?
1) Older engines with more blowby, turn the oil dark faster
2) Engines with gasoline direct injection produce more soot on cold startup and that turns oil dark faster
3)Engines with turbochargers turn oil dark faster. Turbochargers run at very high temperatures and heat and oxidation will turn oil dark faster.
4)Diesel engines turn oil dark faster because they produce more soot than a gasoline engine.
How can you tell if your dark oil requires changing?
There is only one way to know for sure if your motor oil is exhausted and requires changing when it turns dark; an oil analysis. Contact Blackstone Labs to get an oil analysis kit.
©, 2023 Rick Muscoplat