What makes used oil turn dark?

Messages
222
Location
McKinney, TX
I changed the oil in the wife's Xterra this weekend. 6k miles on the truck and 3k on the current fill: 2.5q Pennzoil 5-30 dino plus 1q Pennzoil synthetic. The oil came out rather dark. Darker than the factory fill at 3k and much darker than the GTX I drained out of my 60k mi Ranger, also at 3k. Yeah, I know there are all kinds of variables involved here, but it got me to thinking. I had always assumed that oil darkened primarily from contaminants inside the engine: combustion blowby, varnish, etc. But a new car would have little of either. So what the heck is it that makes an oil turn dark? thx, mr
 
Messages
212
Location
Oregon Coast
Most people think the dark color is in fact dirt, or carbon, Not true, Most of the color you see is due to the oxidation of the additives in the oil due to the heat. I`ve found that with todays hot running engines that the oil darkens guicker, also look at the amount of oil your engine holds, what`s that, 3.5 quarts. I think I would try and find a larger filter so I could have at least 4 if it were me.
 
Messages
5,570
Location
New Zealand
I once saw a demo by a snake oil salesman,forget what it was,but developed in Russia for the space program...blah,blah,blah. Anyway,he got fresh oil and dropped a couple of drops of acid in the test tube,then heated it over a flame - the oil turned black,any oil...we supplied the oils.With his magic ingredient of course it didn't turn black.He said it was the acid build up and heat that turned oil black.
 
Messages
1,759
Location
Elizabeth City NC
It may be heat but most of the time I think it is fuel in the oil. My TR3 runs rich and can turn an oil black in 300 miles. My Toyota and Miata do not run rich and the oil stays clean for sometimes 3000 or more miles. My guess is you have a fuel issue with your new car. This should not happen in a modern fuel injected engine. Do a UOA and you should see high fuel. Then it is dealer time if you are still under warranty.
 
Messages
231
Location
So Cal
I don't think there are any "issues" with the car. I think that the oil darkening is just a natural process in the combustion engine, late modle EFI or carb'd. That said, I don't know why oil turns darker on some motors more quickly than others. Heck, I've had the same type oil (brand and viscosity) stay lighter for longer in the same motor, during the same season, when compared to the batch just before or after. Darryl
 

Al

Messages
19,251
Location
Elizabethtown, Pa
I would have to say it is probably from ring leakage causing more soot particles in the oil. It could be fuel though. An analysis will probably tell you which. I really don't think it is an oxidation issue. When oil turns dark in certain vehicles it happens way before the oil should be significantly oxidized.
 
Messages
9,427
Location
Pensacola & Vero Beach FL
I'd add that there is no one answer to this question. Instead, there are many factors that all conspire to darken your oil (and obviously, no one has yet invented an oil that cleans itself during service inside an engine...). We know that soot does it in diesels. Oxidation of the oil can be a factor, but probably over a longer time, unless your engine is running really hot. Additive breakdown might contribute too. You can imagine this list extending to many other factors, some already mentioned, some not. In the end, color is just one indicator. Not a truth-teller, perhaps a "truth suggestor." I'd say not to worry about color unless something really strange happens (newer gas engine blackens its oil immediately, for example).
 
Messages
810
Location
Greece
I changed my oil(it's the first time i am doing it myself) yesterday because i couldn't stand the black-black colour i had been seeing for the last 800 miles. I know the engine runs rich and also i have to push it hard sometimes but i didn't expect the oil to smell so much like gas. It had to be plenty of gas in it. Mind you that i had been using for 3000 miles Aral's 5W40 group 3 oil which has all the approvals of a modern 5W40 plus GM's-LL-025B diesel approval for extented intervals up to 30.000 miles!! So i am with Al on this. Ring leakage and probably oxidation i think are the main reasons so since it is summer i used something thicker.
 
Messages
2,768
Location
Tn
I don't think color means much at all. GTX used to turn dark in my two engines quickly, but the old Havoline would stay golden. Syntec stays looking clearer than M1, but what does it mean?? Not much. I don't judge by color. I think people worry too much about this and it's rarely an indicator of anything. My last M1 looked black, but the uoa was great. [Big Grin]
 
Messages
4,478
Location
Southern California
In the absence of excessive wear, and assuming the engine is properly tuned, oil darkening suggests to me that the detergent/dispersant package is doing exactly what it gets paid for - holding unfilterable particulates harmlessly in suspension while simultaneously preventing them from congealing with varnish and depositing out as sludge.
 
Messages
238
Location
Monterey Park, CA
If you put a drop of dirty oil on a white towel, you can see the carbon. It's smaller than your oil filter's capability so it goes right thru it. Unless you got a bypass filtration system, it will get darker and darker.
 
Messages
2,480
My observations: 1. dino turns black faster than syth. 2. Thinner oils eg. 5-30, 10-30 turn black faster than higher grade oils eg. 5-40, 5-50, 15-40 etc. 3. A1 oils turn black faster than A3. 4. Lower grade fuel turns the oil black faster than using Premium. 5. Regular use of fuel addatives keeps the oil cleaner looking. The last one is a recent observation that I'm noticing since using gas treatments regularily...but, I'll keep you guys posted. I believe it's a combination of oil and fuel quality and overall engine cleanliness that accounts for the blackening. In addition to the fact that the oil is being baked by the heat of the internal combusion engine temps.
 
Messages
556
Location
Melbourne Australia
I'm not sure if this is correct, but the following comes from the FAQ page on the Castrol Australia website : "Don't worry about the engine oil turning black (it is like washing-up water; it gets dirty, the plates are cleaned)."
 
Messages
2,235
Location
SE MI
In my 1981 T-bird, the oil (any oil) gets black in under 1000 miles. In my 2000 Vic, the oil can get rather dark (M1) but not totally black on the dipstick in about 3000 miles, but once you pour it into a bottle, it's like molasses. This was 5W-30. I believe the reason is that the engine does not get up to operating temperatures to burn off whatever stuff needs to be burnt off (fuel deposits, etc...) The T-bird has a 160F t-stat that is too cold, and the Vic has a 180F t-stat used to help eliminate/reduce detonation. That being said, the 2003 E-250 we have churns out oil that looks about the same as the Vic at about 3000 miles. I have been doing 3000 mile changes with M1 for 1 or 2 changes before stepping to anything higher. ALSO I know many ppl don't sniff their oil... but I noticed that regular driving will cause a fuel-ish smell in the engine oil. If I drag race the car, the oil will have a nice fresh "out of the bottle" chemical smell since you're flashing the oil with extreme engine temperatures and not adequately cooling it down (WOT 6000 RPM shifts then you start idling in the staging lines again OR you shut off the engine). I say, do a few WOT runs on the highway every so often to clean out your motor [Big Grin]
 
Messages
9,427
Location
Pensacola & Vero Beach FL
quote:
Originally posted by yannis: I changed my oil(it's the first time i am doing it myself) yesterday because i couldn't stand the black-black colour i had been seeing for the last 800 miles. I know the engine runs rich and also i have to push it hard sometimes but i didn't expect the oil to smell so much like gas. It had to be plenty of gas in it. Mind you that i had been using for 3000 miles Aral's 5W40 group 3 oil which has all the approvals of a modern 5W40 plus GM's-LL-025B diesel approval for extented intervals up to 30.000 miles!! So i am with Al on this. Ring leakage and probably oxidation i think are the main reasons so since it is summer i used something thicker.
I respectfully suggest that you consider a UOA. As to the fuel, the Mobil-1 in my wife's 2001 Sequoia has been in for about 4,500 miles. It's deep brown colored, but still see-through clear on the dipstick. It has what I perceive as a distinct "fuel-ish" odor about it. I just got back my first UOA results (Blackstone/Dyson), and the fuel in oil is only 0.5%, which is well below the suggested max of 2.0% fuel. I'm thinking that you can have what you perceive as a fuel odor, and still be in great shape, well below dangerous levels. BTW, here's a link to the newly-posted UOA results: Sequoia UOA Results
 
Messages
238
Location
Monterey Park, CA
quote:
Originally posted by metroplex: In my 1981 T-bird, the oil (any oil) gets black in under 1000 miles. In my 2000 Vic, the oil can get rather dark (M1) but not totally black on the dipstick in about 3000 miles, but once you pour it into a bottle, it's like molasses. This was 5W-30. I believe the reason is that the engine does not get up to operating temperatures to burn off whatever stuff needs to be burnt off (fuel deposits, etc...) The T-bird has a 160F t-stat that is too cold, and the Vic has a 180F t-stat used to help eliminate/reduce detonation. That being said, the 2003 E-250 we have churns out oil that looks about the same as the Vic at about 3000 miles. I have been doing 3000 mile changes with M1 for 1 or 2 changes before stepping to anything higher. ALSO I know many ppl don't sniff their oil... but I noticed that regular driving will cause a fuel-ish smell in the engine oil. If I drag race the car, the oil will have a nice fresh "out of the bottle" chemical smell since you're flashing the oil with extreme engine temperatures and not adequately cooling it down (WOT 6000 RPM shifts then you start idling in the staging lines again OR you shut off the engine). I say, do a few WOT runs on the highway every so often to clean out your motor [Big Grin]
Your older car is built with much larger tolerances than current engines. So you get a lot of blowby, and should change your oil more often. Yes I do often clean out my cylinders and pipes with WOT blasts too. A cloud of dust and high ole silver away!
 
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