This may sound a silly question and probably has a real simple answer, but


What is it that turns engine oil black over a period of use?


Cheers



John

 

Carbon

 

It's the various impurities caused by the combustion process held in suspension by the oil. Basically, it shows the oil is doing its job by not allowing them to settle and build sludge. These particals are EXTREMLY small - maybe only a couple of microns and will pass an oil filter. They are not generally harmful, but they will build up over time and oil changes should be regular and if you do short journeys, frequent.
JohnD

 

Always wanted to know that myself

 

if the oil is black its been in far too long! Its carbon either from combustion or what was was picked up in the oil ways from old carbonised oil.
when you see new oil it looks almost clear with a gold tint. When its gold with a brown tint change it.
From many years of experience! New oil feels nice on your fingers. Smooth like silk. After a while you can do this and feel the oil "squeak" on your fingers. Its not always dirt that does this. Oil breaks down under high tempratues and even if it looks clean may be useless!

 

You want//need to get our more!!

 

thanks

Some interesting posts


I thought it was carbon, was just checking


john

 

I think you meant to say black and thick, just black aint a problem, that shows its working properly and cleaning your engine
Thick and slow on the other hand shows it definately needs changing
Though my TD changes black even after an engine flush and 5 mins running!
but my old 100k mondeo had lovely golden coloured oil after 10k's worth of driving! (and yes it was the stuff put in 10k miles before )

Tony

 

If the oil is black so quickly change it again. Cleaning is fine but if the oil pulls the dirt out and you leave it in all that happens is that the "dirt" goes back to where it came from. When i get a new car its not uncommon for me to change the oil 3 times in the first 6 months. After that I go for the normal service duration and the oil is clean.

 

i know that the heat does cause the characteristics to change over a period of use, when its said that its breaking down, what exactly does this mean, molecules separating????

 

The other thing to look out for is smelling heavily of petrol. This indicates fuel dilution, the biggest killer of oil, any quality.

It lowers the viscosity and even 8% fuel dilution can reduce a 15w-50 to a 5w-30 in less than 1000 miles.

If it smells badly of petrol, change it and sort out the overfuelling.

Cheers
Simon

 

Mine is black a few hundred miles after the 60k service

 

Breaking down is known as shearing and is when the multigrade plolymers are unable to stop the oil thinnng beyond its viscosity spec.

A more detailed explanation is here and I posted it before somewhere here:

Why do oils lose viscosity with use?

Viscosity Index Improvers.

An oils viscosity will decrease as the engine temperature rises. Viscosity Index Improvers are added to reduce this thinning. They are a key addative in the production of multigrade oils.

VI Improvers are heat sensitive long chain, high molecular weight polymers that minimise the viscosity loss of the oil at high temperatures. They work like springs, coiled at low temperatures and uncoiling at high temperatures. This makes the molecules larger (at high temps) which increases internal resistance within the thinning oil. They in effect "fight back" against the viscosity loss in the oil.

"Shearing"

The long chain molecules in VI Improvers are prone to "shearing" with use which reduces their ability to prevent the oil from losing viscosity. This "shearing" occurs when shear stress ruptures the long chain molecules and converts them to shorter, lower weight molecules. The shorter, lower weight molecules offer less resistance to flow and their ability to maintain viscosity is reduced.

This shearing not only reduces the viscosity of the oil but can cause piston ring sticking (due to deposits), increased oil consumption and increased engine wear.

Like basestock quality, VI Improvers also vary in quality. As with many items the more you pay, the better the finished article and more expensive, usually synthetic oils are likely to incorporate better VI improvers. All other things being equal the less VI improver an oil contains, the better it will stay in grade by resisting viscosity loss.

Which oils require more VI Improvers?

There are two scenarios where large amounts of these polymers are required as a rule.

Firstly in "wide viscosity span" multigrades. By this I mean that the difference between the lower "W" number and the higher number is large for example 5w-50 (diff 45) and 10w-60 (diff 50) are what is termed as "wide viscosity span" oils.

Narrow viscosity oils like 0w-30 (diff 30) or 5w-40 (diff 35) require far less VI Improvers and therefore are less prone to "shearing".

Secondly, mineral and hydrocracked (petroleum synthetic oils) require more VI Improvers than proper PAO/Ester (Group IV or V) synthetic oils as they have a higher inherent VI to begin with, this is due to differences in the molecular structure of the synthetic base oils compared to mineral oils.

It is a fact that many synthetics require significantly less VI Improver to work as a multigrade and are therefore less prone to viscosty loss by shearing.

Cheers
Simon