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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 Jun 7 '12 at 20:19

5 Answers 5

up vote 9 down vote accepted

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 Jun 8 '12 at 20:58
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Upvote. The keyword is the "diesel". Petrol engines oil keeps clean for much longer time –  Krom Stern Jun 9 '12 at 5:23
    
Heh, turbo petrol engines with forged pistons tend to dirty oil really fast too... –  Brian Knoblauch Dec 6 '12 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 Rinea Aug 27 '13 at 13:04

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 Jun 13 '12 at 3:39

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. –  Krom Stern Jun 8 '12 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 Jun 8 '12 at 10:28
    
Good point, you right. –  Krom Stern Jun 8 '12 at 10:30

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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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 Jun 7 '12 at 18:14
    
After reading more about diesel oil changes I'm convinced that in diesel engines oil gets dark soon after change. –  Krom Stern Jun 9 '12 at 5:21
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In gas circumstances this is true, but not for diesels. –  Parker Jun 9 '12 at 23:50

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