Engine oil unusually black

Vinman

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So I change the oil in my 06 TJ 4.0L every 3,000 miles and for the last 1,000 or so before changing it is so black it looks like it has many, many more miles on it. Jeep sees mainly highway miles and not much idle time at all. I don’t remember any of my previous TJ’s turning the oil this dark. Jeep has about 70,000 miles on it and I’ve owned it for the last 10,000 or so. Not sure of the PO’s maintenance. Oil used is Rotella 15W40.
Is this normal and I just didn’t notice it in previous Jeeps?
 

UnoBurro

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Apr 28, 2018
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Black on the dipstick or black looking once drained from the pan? My stock 2001 Sport I noticed since new would darken the oil quicker than my V6 Ford Ranger engine. I've also read posts stating you can not judge the life of the oil by color alone these days due to all the cleaning detergents doing their job? I've never used Rotella oil before but wouldn't a 15W-40 weight be a little thick for winters in Canada?
 
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RMETeeJay

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That's a diesel oil, although that in itself wouldn't harm anything. Dark oil just means that soot particles are suspended in it and you are changing it at the recommended interval for the majority of drivers ('Schedule B' in the manual).

However, you're using the wrong weight oil for the engine. @UnoBurro is right that you're running way too thick of an oil. You should use 10W-30 or 5W-30 in the 4.0L using the chart from your manual.
 
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mots

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I've never used Rotella oil before but wouldn't a 15W-40 weight be a little thick for winters in Canada?
Agreed. A 5W30 should be used in colder climates (per service manual), or at least a 10W30 synthetic that flows well when cold. Rotella is likely fine, but not that weight.
 

Goatman

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Agree with dark oil is just oil detergents doing their job. Every oil has it's own proprietary detergent package, same as gas. Switching brands will allow a breakdown of whatever the prior oil was allowing to build up. 15-40 way to thick. Can probably says for diesels right on it. 10W or 5W -30 for the 4.0. Some folks like to run some diesel oil for the higher zinc package to help with tappet wear, but just use a high mileage oil and you get improved protection that way. Using a thicker oil can actually increase wear in the engine.
 

toximus

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It'll get black within a few miles of use. It's normal.

If you are concerned you can send a sample in for analysis.
 
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Vinman

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Sep 20, 2018
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Calgary, Alberta
Thanks guys, I’m not terribly concerned about the thicker weight oil since the Jeep very, very seldom sees bitter cold starts. It lives in a heated garage and doesn’t go out much during the Winter months. That being said, I think I will step it back to a lighter weight oil next change.