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I took my 2002 Jetta TDI wagon to a neighborhood shop about 10 miles, 24 hours ago. They supposedly changed the oil, but the color is very dark, almost black. Definitely not light-colored/transparent. The oil change was performed along with other work (replacing alternator & brake pads).

I'd run it back right away without a thought, but I'm wondering if this is normal for a 10-year old diesel car with 175,000 miles on it. Last oil change was made around 10,000 miles ago. The engine is original, too.

Not sure if this is relevant, but I always have 5% to 99% biodiesel and Diesel Kleen in the fuel tank.

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    It might depend on how dirty it was before the change and if the engine was warm when they drained it. If the oil was exceptionally black and changed when cold there might be enough left to darken the new oil. Even in the worst case though it should look lighter than when you brought it in.
    – mikes
    Commented Jun 7, 2012 at 20:19
  • This thread has been very helpful. Just bought my first diesel car and although only done 15000 miles and 3000 since the last oil change the oil is very black. Was shocked at first as all my previous cars were petrol and oil was always light brown. I did think the dealership had lied to me but it seems black oil in a diesel engine is the norm. Thanks for the insight.
    – user33019
    Commented Oct 15, 2017 at 9:10

7 Answers 7

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It's a Diesel, which means that you usually have a high detergent oil in an engine that dumps combustion by-products like soot into the oil as part of its normal operation. Given the age of the vehicle I'm not surprised that the oil has noticeably darkened after 10 miles - one of the older Diesels I owned a while back did that during the time it took to run up the engine after the oil change and fill the oil filter.

Also, when changing the oil, you'll never get all the oil out of the engine so the new engine oil will mix with a little oil that already contains combustion byproducts. This is nothing to worry about (it's usually a very small amount of oil that is left in the engine), but it'll also lead to oil darkening fairly quickly.

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  • This is exactly what my mechanic said.
    – Adam Monsen
    Commented Jun 8, 2012 at 20:58
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    Upvote. The keyword is the "diesel". Petrol engines oil keeps clean for much longer time
    – Kromster
    Commented Jun 9, 2012 at 5:23
  • Heh, turbo petrol engines with forged pistons tend to dirty oil really fast too...
    – Brian Knoblauch
    Commented Dec 6, 2012 at 14:43
  • I second this opinion too. I've owned Diesel cars since 2001 (TDIs) and it's perfectly normal. No worries whatsoever.
    – Andrei Rînea
    Commented Aug 27, 2013 at 13:04
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To add to what others have said, diesel lube oil (the code starts with "C", like "CJ") is formulated to hold lots of soot, and after 10,000 miles, there probably was a lot of soot in the remaining oil that got mixed in with the new oil.

When changing your own, never put "S" (for "spark") rated oil in a diesel engine!

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  • Oh, nice! I just learned that the "C" is for compression, too.
    – Adam Monsen
    Commented Jun 13, 2012 at 3:39
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A good trick is to put a small mark on the filter, e.g. a scratch or bit of marker pen that you can identify, but isn't obvious. When you get the car back, check to see that the mark is gone (i.e. they have changed the filter). If the mark is still there, you still have the old filter, and you know you've been screwed...

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  • Usually old filter has enough dust on it to clearly see a difference between it and a shiny new one. So taking a look at the filter may give you some hints post factum.
    – Kromster
    Commented Jun 8, 2012 at 9:51
  • Yeah, but it is fairly easy for an unscrupulous garage to clean the dust, oil etc off an old filter, hence adding a more permanent mark...
    – Nick C ♦
    Commented Jun 8, 2012 at 10:28
  • Good point, you right.
    – Kromster
    Commented Jun 8, 2012 at 10:30
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After changing Diesel oil in an engine it will always look blackish and never a golden colour after only a very! short period of time. This is because the old oil residue left when the oil is changed will always retain a certain amount of contamination and cause the discolouration. You will never remove all of the old oil even if the oil is hot when removed. It is designed to naturally adhere to internal component this his how it protect your engine.

Petrol oil will always be cleaner due to the difference in the way the engine function any build up of contaminants is reduced considerably.

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Hmm, I've known oil to go black very quickly in older diesels, but not for it to be essentially instant. The higher mileage of yours may be a factor, but unless they did it very quickly without letting the sump fully drain, I'd have expected to see some change.

If you're super bothered, change it again yourself - catching what's in there already in a clean pan, so you can decant it into the soon-to-be-almost-empty bottle of new stuff, in case it turns out to be possibly reusable, and letting it drain out as long as you can stand (or until it stops, whichever is sooner - then flushing through a gulp of the new stuff, if the spec doesn't say to use the entire bottle) - and see if it remains nasty the second time. It's still not conclusive if it comes cleaner, maybe you just flushed out the last of a heavy dose of residual sludge, but if it stays dark you may at least be able to trust your garage a bit more. Hopefully the differences with new brake pads and a new alternator are a bit easier to discern...

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First off, unless the engine is completely flushed, there will be some residual dirty oil still inside. Then there are the normal by-products of an internal combustion engine: soot, metal wear particles, old oil residue breaking down, etc.

The oil filter elements are designed to collect as much of this detritus as possible without hindering free oil circulation through the engine (dirt = bad, low oil flow = worse bad). Perform regular oil & filter changes as per mfr mileage recommendations.

A coworker who was a car fanatic changed his oil & filter every 3,000 miles. BUT he changed his oil filter EVERY MONTH. His rationale was that he was throwing away the dirt the filter collected on a regular basis. He had 375,000 on his Eddie Bauer edition truck, still going strong. Tough to argue that.

Side point: on our rotary screw air compressor (not internal combustion, I get that) the recommendation was to replace the oil filter monthly for regular PM and submit oil samples for analysis. SO regularly 'taking out the trash' seems like a reasonable thing to do. Pretty cheap, too, more hassle than cost for the backyard mechanic. Worth trying.

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  • I'm not sure how this answers the question?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 9, 2016 at 14:37
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Fresh oil is a light tan in color. There shouldn't be enough old oil left to make the oil on the stick dark. Either they forgot, or you got screwed.

(ah, ha! I've never owned a Diesel).

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  • Now I'm not so sure. I just brought it back in. Mechanic took a look and said the color is dark but normal, next time they'll do a "purge"--put in some compound to clean out the engine. FWIW, oil change was $20 labor, $9 filter, $38.80 for 5 qt 5W-30 synthetic.
    – Adam Monsen
    Commented Jun 7, 2012 at 18:14
  • After reading more about diesel oil changes I'm convinced that in diesel engines oil gets dark soon after change.
    – Kromster
    Commented Jun 9, 2012 at 5:21
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    In gas circumstances this is true, but not for diesels.
    – Parker
    Commented Jun 9, 2012 at 23:50
  • @AdamMonsen Do NOT do a "purge" on a high-mileage engine. If the discoloration is due to left-over contaminants, live with it. An engine flush on a high mileage engine is a very, very Bad Idea.
    – 3Dave
    Commented Jul 2, 2015 at 1:55

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