Why Does Some Oil in Engine Smell "Worse"?

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So this has been on my mind the last few weeks and I'm curious. I have smelled over used oil before - always on other people's vehicles. I have smelled oil that has been run a reasonable interval ... 5000-7500 miles.

A few weeks ago I decided to replace the rear main seal and oil pan gasket on my Jeep 4.0 (that's a whole other fuster cluck). The seal wouldn't come out and I had to loosen the other six main bearing caps.

This, of course, allowed the oil passages within the engine to drain out completely.

The oil that came out smelled HORRIBLE. The only time I have ever smelled oil like that was on a vehicle that had gone >15000 miles without an oil change. It was also very dark black.

The oil that was in the pan was "regular" smell and color. It was only in for 2000 miles or so.
 
Whenever I've overhauled an engine, it always seems to small nastier than a normal oil change. Probably due to some contaminants sticking to metal and being broken loose.

my .02, let's see what others have to say...
 
I imagine it was the complete draining of the oil passages that made the difference, probably higher in contaminants and such than what you would get out on a normal oil change.
 
I noticed that Chevron Supreme 10w40 stunk to high heaven when it was fresh and new.

Best way to describe it would be like oil from 10/15 years ago.

And I have used semi synth and full synth of many different brands.

I assumed it was just due to the formulation.
 
I have a boat load of 5w20 Supreme here and now you got me wondering
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Originally Posted By: bigjl
I noticed that Chevron Supreme 10w40 stunk to high heaven when it was fresh and new.

Best way to describe it would be like oil from 10/15 years ago.

And I have used semi synth and full synth of many different brands.

I assumed it was just due to the formulation.
 
Happy birthday Merk, and I've changed a lot of QS over the years, and never noticed anything "wrong" with it. The only thing that really bugs me is differential oil. Maybe the fuel dilution in the old F-150 curled my nose hairs over the years.
 
I believe it was oil that had been wherever it was for quite a while. I have never run quaker state in this Jeep. Not that I think it's a bad oil ... I just have never used it.
 
The horrible QS of memory for those of us that remember the sludged engines of the 80's is just that...a memory since Shell overhauled the brand. Heck, most oil in the 80's was junk by modern standards.

I have had excellent luck with QS synthetic oils in my cars.
 
I remember Pennzoil conventional before it became Sopus smelled like rotten eggs fresh out of the container to me.
 
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