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I checked the oil level on my car that had oil changed 2000 km ago (the car doesn't seem to consume any oil, so I don't check it every 1000 km). The oil color was in an intermediate state between new oil and used black engine oil.

Is it possible to use the oil color as an estimate of the engine condition? You could measure the time it takes for the oil to turn black, and then use that measure as an engine condition estimate.

Of course, this measure won't be completely objective, as it's hard to exactly define what "black" means. However, it could be perhaps used to identify an engine having a major wear problem: if the oil turns black 10x faster on one car when compared to another, the first car may have a major engine problem.

I know there are oil analysis laboratories that can do objective measurements. Could oil color be one of many such objective measurements?

This question might provide a partial answer: Can you mix black oil caued by poor air fuel ration with new oi? ... as apparently air fuel ratio that is off can color the engine oil black.

  • You are probably better measuring the vibrations from the engine and tracking that over time... challenging though in many ways but it is being done for some equipment... – Solar Mike Jul 22 '18 at 9:47
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    Oil color is a poor method to evaluate an engine. There are labs that will analyze a oil sample and give a good evaluation . – blacksmith37 Jul 30 '18 at 14:36
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In a word, no.

Why does oil turn black?

Soot particles are tiny (<1 micron), and they darken the oil. Oil turning black depends on so many factors - fuel quality, combustion quality, usage patterns, engine condition, care taken in the last service and more.

Fuel quality

If your fuel is lower in grade, combustion is less complete. More expensive fuels also contain additional lubricants and cleaners which may reduce the amount of soot in the oil.

Combustion quality

If you have poor ignition components, your oil will turn black quicker. Worn or incorrectly gapped spark plugs will have poor combustion and lead to black oil.

Usage patterns

Heat cycles also cause oil to turn black. If a car does lots of town journeys, it doesn't get up to operating temperature so the tolerances may lead to more soot getting into the oil.

Engine condition

This is the factor you're worried about - worn piston rings for example. While this is definitely a factor, there are other ways to figure out how worn an engine is - oil consumption, fuel consumption etc. are better measures. Check your oil and fuel consumption are within manufacturer limits.

Care in servicing

This isn't to do with engine servicing - I'm assuming the engines are serviced according to manufacturer recommendations. There is a huge difference between "suction" removal of the oil and replacing it (jiffy lube etc. - cheap and quick services), and a proper drain and refill service. The drain and refill will get much more of the black oil out, so the new oil will be cleaner for longer.

Personally, I remove as much of the oil as possible (drain oil, pour a little fresh oil through the top and letting it drain, and remove as much as I can get from filter housings etc. using suction at the end before putting it back together and refilling). This care probably means I'm replacing 80-90% of the oil, compared to as little as 50% for the suction/refill method. It takes time, care and attention to do this though.

All these factors together mean that it's difficult to attribute oil blackening reliably to any one of these, without being methodical and scientific about how you're measuring and what variables you're changing.

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Some engines have more blow-by than others. You're probably doing fine. One possible cause of premature oil contamination is inadequate crankcase ventilation. Check your PCV valve (ball should rattle) if accessible or check for air flow into the crankcase from the air cleaner at idle.

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I'm not sure if it's much a sign of engine condition with a gasoline/petrol fuelled car (I've had several of different age and wear levels, and a bike to boot, and the oil seems to darken at about the same rate in all of them ... except the one with a notoriously poor level of sealing between oil and coolant even with brand-new gaskets, where it progressively turned to mayo)... but if you're checking it in a diesel fuelled one, it's no sign at all. It'll go black within a couple thousand miles pretty much universally, because they just make much more soot.

In that case, if you're changing it at least as regularly as the manufacturer's recommendation for your driving conditions (you could cut that down to as little as half if you're paranoid, but anything less is pointless), and using good quality oil, and the engine seems to otherwise be in good health (no funny noises or vibrations, EGR is clear and working correctly, no broken sensors or warning lights), then there shouldn't be anything to worry about regardless of the colour.

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