Yet another, what causes oil to turn black thread?

[regalman]

First off, I did do some searching, seemed like several poeple have differing opinions on what turns oil black. The reason I ask is because someone made some comment like "I looked at the oil and it was so dirty that I immediatly had to swap it out" For some reason the use of 'dirty' in regards to someone looking at black oil is now one of my newest pet peaves. So what exactly causes oil to turn black? Is it mearly heat cycles? Does it really have anything to due with combustables being suspended in it? People made comments about dye, is the dye simply depleted somehow? So, if it has to due with heat... could I take a quart of oil, put it on my stove, carefully heat it up to operting temp for an hour or so a few times and have it turn black?

 

[moribundman]

Well, from my experience with paint pigments I can assure you that even a tiny amount of carbon black, which is similar to soot and a common used pigment, has a tremendous capacity for coloring comparatively large quanties of resin, oil, water, etc. Carbon black is (chemists, correct me if I'm off!) a byproduct of incomplete combustion, so my guess would be that it's the carbon black that colors the oil. As I said, a tiny amount has a lot of coloring power.

 

[Gary Allan]

Tony (Bio-T) posted something on the effects of heat and some additive packs. That is, a certain amount of darkening is just what happens when the oil goes through so many heat cycles. Naturally, suspended solids are going to make your oil less translucent.

 

[JAG]

It's two things mostly with a possible third: 1. Carbon black suspended in oil, as Mori said 2. Heat - different oils darken differently from heat; yes heating virgin oil will darken it eventually 3. IF there is sludge and a good oil is put in that dissolves some of it, that will color the oil too We all have probably seen newly put in oil look pretty dark almost right after an oil change. That's due to suspending the carbon from previous fill. Oil doesn't turn dark from heat that quickly. Ever see oil from a diesel engine? It looks like black ink from all that soot those engines generate. Carbon from gas engines is so small that it doesn't cause significant wear like it can in diesel engines. So someone saying oil looks dirty and changing it really early is wasting oil in most cases.

 

[Benzadmiral]

From what I've heard from die-hard MB diesel fanatics, you can take the finest rated-for-diesel oil and pop it into a perfectly-running diesel engine, and within 20 minutes the oil will appear black. Apparently the diesel-rated oil suspends the soot, a natural byproduct of the diesel combustion process, in it -- that's why you want a "C"-rated oil for your diesel.

 

[DriveHard]

quote:

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Originally posted by Benzadmiral: From what I've heard from die-hard MB diesel fanatics, you can take the finest rated-for-diesel oil and pop it into a perfectly-running diesel engine, and within 20 minutes the oil will appear black. Apparently the diesel-rated oil suspends the soot, a natural byproduct of the diesel combustion process, in it -- that's why you want a "C"-rated oil for your diesel.

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You can do the same with a VW TDI, and by the time you back it down off the ramps its black as night.

 

[gedcruise]

I have had two W123 240Ds. In both, which always got 3K oil and filter changes and had huge filters, the oil always came out jet black. It also had a very distinctive scent. You haven't seen dirty looking oil until you've seen drain oil from a diesel.

 

[ekpolk]

Shoot, I'd look at this characteristic of a diesel as a convenience feature. With my vehicles, for the first couple days post oil change, the oil is so water-clear that it's hard to see the level on the stick.

 

[MN Driver]

I've had the experience of having oil look new and then have it suddenly change a few shades darker around the 2500 mile mark and then over the rest of a 7500 mile OCI it never got any darker, I've noticed that with a few of the cars I take care of and it's always got me scratching my head as to why it doesn't continue getting any darker. Then again I've seen oil get darker with much less mileage in cars that had a decent amount of varnish under the valve cover and I am sure it has something to do with solvency of oils' past. If I was dealing with a sludger engine I would probably go by the look of the oil for a few short OCI's to judge when to change it. I know that the first Mobil 1 with my Prizm became dark within 1,000 miles and the second Mobil 1 run wasn't as dark until well over twice the mileage. I ran the first run to 6,000 miles as it was the second run with synthetic(Could have been a Grp3 though) otherwise it would have probably been dumped at 3,000 for the first run. The only explaination for this activity was that it was cleaning a small amount of varnish or sludge out of the engine, which reminds me I need to pull my valve cover and take a picture this weekend of what is probably a pretty shiny engine.

 

[ITSMR2U]

I once left the oil filler cap off on my VW Jetta diesel. Really bad mistake!

 

[MC5W20]

Has anyone considered its the gremlins doing it?