Does oil get darker because of dirt?

B

BrianApp

Joined
Jun 22, 2016
Messages
62
Location
MI
I was wondering what causes oil to darken as it is used. Is it, dirt in suspension, a sign of the additives being consumed, or just a normal change as the oil is exposed to the combustion environment? I have a 350,000 camry and change the oil consistently at 3,000. The oil starts to darken fairly quickly and comes out at the OCI almost looking black. At first I thought this was just because of an oil engine and the oil getting dirty but why should the oil be getting any more dirt in it just because the engine is old? There are no holes in the air intake system and the air filter is clean and a good brand. Then I started thinking why would the darkening of the oil have to be from suspended dirt and contaminants.
So, does oil darken with use from suspended contaminants or is it just a normal chemical reaction when exposed to the stress of the combustion process?
Also, does the darkening relate directly to the need for it to be changed or is its color irrelevant as an indication of the life left in the oil?
 
Oil darkens from combustion byproducts, namely soot, and I guess some heat effects too. I wouldn't call it dirt unless your air filter is failing you. Color doesn't mean much as to oil life, unless being clear shows that it is brand new.
 
BrianApp said:
...
So, does oil darken with use from suspended contaminants or is it just a normal chemical reaction when exposed to the stress of the combustion process?
Also, does the darkening relate directly to the need for it to be changed or is its color irrelevant as an indication of the life left in the oil?
Click to expand...

Both but in most cases it's probably the former unless you're changing oil on an engine that hasn't been maintained well...
 
You might have a cylinder or two running rich, like maybe from a leaking injector. Other than that, 350,000 somethings might just be from ring blowby.
 
Oil gets dark because of oxidation. Contaminants like soot, fuel dilution, water dilution, dirt, etc... will all accelerate oxidation. Heat also accelerates oxidation. This isn't concrete though. There's some ashless antioxidants that turn dark from UV exposure (from combustion events) making the oil appear black when it actually isn't. I don't think that's your case though.

Knowing how dark the oil is in virgin form is important too. There's some oils that are rather dark right out of the bottle because they have a hefty add pack with like 1.5% sulfur.
 
BrianApp said:
So, does oil darken with use from suspended contaminants or is it just a normal chemical reaction when exposed to the stress of the combustion process?
Click to expand...
When you change it, are you draining out as much of the old oil as possible? If not, whatever was left is darkening the new oil you put in. Just as a test, the next oil change, do one back to back. i.e. change the oil and filter, drive it 5 or 10 miles, then change it again and drain the filter. Check in 3,000 miles how much cleaner it looks.
 
The thing that pushed me over the edge to switch from dino to synthetics was in part appearance after an oil change.

I've been changing my own oil more often than not, since I could drive now going on 3 decades ago. Conventional oil around 3000 miles has always been tar black at a drain. This suggests to me that it's spent, it's absorbed nearly all of the dirt and heat that it is designed to do. BTW, if you watch testers "cook" new oil, it turns black.

About a decade or so ago, I used and drained a synthetic at something like 5000 miles. It still looked new, retaining its golden caramel color. That really impressed me, suggesting it still had a lot of life remaining. The fact that all professional drivers use synthetics is quite telling. In the long term, synthetics seem to me to more cost effective.

Now, I know that my observations are not scientific. But it's a data point. If new oil is golden, I assume that when it turns black it has lost some of its lubricating properties. I cannot explain if it's due to heat or dirt or altered chemistry, but something is happening to uniformly turn the oil from golden to black... Conversely, synthetics don't seem to turn black until a LOT longer miles and time. But the transformation occurs, and when it occurs, I likewise assume it's losing or lost its lubricating properties.
 
Also, does the darkening relate directly to the need for it to be changed or is its color irrelevant as an indication of the life left in the oil?

In an otherwise healthy motor, the color of oil is irrelevant as an indication of life left. I do NOT speak from a perspective of assumption or "guesswork." I have spent a small fortune on UOA over a time span of several years. The "color of oil" is a topic on BITOG which never seems to go away and has been discussed ad nauseum.
There are several things which can be determined from the color/smell of oil; fuel dilution, coolant leaks, possible blown head gaskets, etc., but I started out by stating "in an otherwise healthy motor," so I'm assuming that anyone concerned about the color of their oil is not talking about "mayonnaise" or fuel dilution or "dirty appearance."
You CANNOT tell the TBN (remaining service life) or particle count or really anything that a test lab will tell you.
You CANNOT tell how many ppm iron, silicon, copper, boron, etc., (wear metals or contaminant metals) by the color of oil.

As Trav has well stated, if you're trying this on a diesel, you're going to change it daily.
Of course, if you're in the classification of BITOGGERS who want their oil "golden" in appearance, you can change your oil every weekend if you like.

I have sent in many samples for analysis of "black" oil, which has plenty of TBN left, low levels of wear and contaminant metals, and the test lab results were "suitable for continued use." Full disclosure: I use Amsoil SS (full synthetic and "boutique" oil). My suggestion to you is to send in for UOA a sample that has 3K on it; but, with 350K on the engine, why do you doubt anything now? Why not about 275,000 miles sooner? Obviously, whatever you're doing has gotten you this far....I wish you well. Live long, prosper, and stay oily.
 
Last edited:
TexasPlumber said:


this guy heated motor oil and it got dark
Click to expand...

Hey I've got a couple cases of that 😆
20210403_145353.jpg
 
Ihatetochangeoil said:
Also, does the darkening relate directly to the need for it to be changed or is its color irrelevant as an indication of the life left in the oil?

In an otherwise healthy motor, the color of oil is irrelevant as an indication of life left. I do NOT speak from a perspective of assumption or "guesswork." I have spent a small fortune on UOA over a time span of several years. The "color of oil" is a topic on BITOG which never seems to go away and has been discussed ad nauseum.
There are several things which can be determined from the color/smell of oil; fuel dilution, coolant leaks, possible blown head gaskets, etc., but I started out by stating "in an otherwise healthy motor," so I'm assuming that anyone concerned about the color of their oil is not talking about "mayonnaise" or fuel dilution or "dirty appearance."
You CANNOT tell the TBN (remaining service life) or particle count or really anything that a test lab will tell you.
You CANNOT tell how many ppm iron, silicon, copper, boron, etc., (wear metals or contaminant metals) by the color of oil.

As Trav has well stated, if you're trying this on a diesel, you're going to change it daily.
Of course, if you're in the classification of BITOGGERS who want their oil "golden" in appearance, you can change your oil every weekend if you like.

My suggestion to you is to send in for UOA a sample that has 3K on it; but, with 350K on the engine, why do you doubt anything now? Why not about 275,000 miles sooner? Obviously, whatever you're doing has gotten you this far....I wish you well. Live long, prosper, and stay oily.
Click to expand...

Then please explain what chemical reaction is occurring when you cook oil to turn it from gold to black? Your engine is ultimately "cooking" the oil which is changing the properties, or rather worsening the design properties. In the process it's turning black.

Do you think that chemical reaction is
A: Improving the oil's viscosity and ability to resist thermal breakdown, or
B: Degrading the oil's viscosity and ability to resist thermal breakdown?

I'm inclined to believe when oil is so hot it's turning black (from being subjected to heat), that some negative thermally is happening to weakening the bonding and lubricating and HEAT ABSORBING ability. Otherwise if black was optimal, it would come at that chemical makeup. Would it not?

IOW, if Valvoline determined that before shipping their oil, a good cooking to change the chemical makeup to improve it is necessary and gives a better oil product, don't you think they'd do just that?

To what degree, not sure. But when a dino oil has turned black at 3000 miles, that's a data point. When it takes a synthetic in the same motor and same conditions eliminating all reasonable variables, some 7000 miles to turn black, that's another data point.
 
MrLemonade said:
Then please explain what chemical reaction is occurring when you cook oil to turn it from gold to black? Your engine is ultimately "cooking" the oil which is changing the properties, or rather worsening the design properties. In the process it's turning black.

Do you think that chemical reaction is
A: Improving the oil's viscosity and ability to resist thermal breakdown, or
B: Degrading the oil's viscosity and ability to resist thermal breakdown?

I'm inclined to believe when oil is so hot it's turning black (from being subjected to heat), that some negative thermally is happening to weakening the bonding and lubricating and HEAT ABSORBING ability. Otherwise if black was optimal, it would come at that chemical makeup. Would it not?

IOW, if Valvoline determined that before shipping their oil, a good cooking to change the chemical makeup to improve it is necessary and gives a better oil product, don't you think they'd do just that?

To what degree, not sure. But when a dino oil has turned black at 3000 miles, that's a data point. When it takes a synthetic in the same motor and same conditions eliminating all reasonable variables, some 7000 miles to turn black, that's another data point.
Click to expand...
You know the answers to your own questions. To what degree, only you know. If in doubt, why don't you spring for UOA like I do? The test lab will answer your questions for your engine and driving habits. I cannot. If you want to change your oil when it gets black, you have my blessings; I drive too many miles to deal with that. Black oil does not mean unserviceable. You do you and I'll do me. Peace.
 
Ihatetochangeoil said:
You know the answers to your own questions. To what degree, only you know. If in doubt, why don't you spring for UOA like I do? The test lab will answer your questions for your engine and driving habits. I cannot. If you want to change your oil when it gets black, you have my blessings; I drive too many miles to deal with that. Black oil does not mean unserviceable. You do you and I'll do me. Peace.
Click to expand...
@Ihatetochangeoil

How much does a UOA cost?
 
MrLemonade said:
@Ihatetochangeoil

How much does a UOA cost?
Click to expand...
Blackstone is $30 I believe; extra $10 for TBN. I also have them do particle counts per iso 4406 which is an extra $20. Here is Blackstone's price list: https://www.blackstone-labs.com/tests/tests-price-list/ I do not have blackstone do TBN for me because I also send samples to Polaris test lab, $28 for analysis which includes TBN. I NEVER send samples USPS because of their lack of services; it may take two weeks or more to get a sample from south St. Louis to Indiana (a 6 hour drive). I always use UPS for $11 because they get it there overnight, next day delivery.
Polaris labs do reports in color coded charts which I like, and they include TBN in their cost of $28; but their particle count machine broke and they're not fixing it. I run bypass filtration and I may go 30,000 miles before an oil change, so particle counts are very important to me. https://www.machinerylubrication.com/Read/353/particle-counting-oil-analysis
In my honest opinion, you don't need "regular" oil analysis. I would guess you're just interested in seeing how long you feel you're comfortable running your own oil. If you feel I've left anything unanswered, don't hesitate to PM me or ask. I'm kind of a black sheep here on BITOG; I follow test lab advice over the armchair tribologists around here.
 
Ihatetochangeoil said:
Blackstone is $30 I believe; extra $10 for TBN. I also have them do particle counts per iso 4406 which is an extra $20. Here is Blackstone's price list: https://www.blackstone-labs.com/tests/tests-price-list/ I do not have blackstone do TBN for me because I also send samples to Polaris test lab, $28 for analysis which includes TBN. I NEVER send samples USPS because of their lack of services; it may take two weeks or more to get a sample from south St. Louis to Indiana (a 6 hour drive). I always use UPS for $11 because they get it there overnight, next day delivery.
Polaris labs do reports in color coded charts which I like, and they include TBN in their cost of $28; but their particle count machine broke and they're not fixing it. I run bypass filtration and I may go 30,000 miles before an oil change, so particle counts are very important to me. https://www.machinerylubrication.com/Read/353/particle-counting-oil-analysis
In my honest opinion, you don't need "regular" oil analysis. I would guess you're just interested in seeing how long you feel you're comfortable running your own oil. If you feel I've left anything unanswered, don't hesitate to PM me or ask. I'm kind of a black sheep here on BITOG; I follow test lab advice over the armchair tribologists around here.
Click to expand...
Thanks for the detailed response.

Makes more sense IMO to just do my $20 oil change at sensible intervals...
 
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