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Why does my oil turn black just after a service of my diesel?

So your car has been in for a scheduled service and running beautifully after the exercise. A week later while you are washing the car, you pop the hood, and causally have a little look around, just to make sure mind you, that your mechanic has actually done what he said he had.
After all, looking under the hood or kicking a tyre is almost part of our DNA now. Along with being deeply suspicious of your mechanic.

Now, even a decade ago you could really have a nose around – checking the filters and spark plugs, peering into the alternator and pushing and prodding whatever else you could get your fingers into.
Modern vehicles, however, offer limited access to this playground.
But the dipstick is still reachable, so oil check it is.
You juggle the stick around, pull it out and peer at it intently, hoping to impress any on-lookers with your mechanical prowess.
The oil level is perfect, but hey, it doesn’t look as clean as you think it should be.

Now before you bang the hood shut and storm off to accuse your mechanic of every indiscretion under the sun, hold on to your horsepower for just a second.
If your car is a diesel, black oil, even this soon after a service is perfectly normal.
The oil in your engine is there for three main reasons – to lubricate the moving metal parts reducing friction, to help in cooling by transferring the heat from the metal components to the sump and to clean the engine of the carbon deposits that can hinder performance.
It is the latter that is behind your oil turning black.
Diesel combustion engines produce much more soot (partially burnt fuel) and sludge as part of their normal operation than their petrol counterparts.

The modern trend for direct engine systems compounds the problem because while the higher fuel injection pressures in newer diesel engines produce lower exhaust emissions, it increases the production of soot.
The soot forms in cooler parts of the combustion chamber until it impinges on the cylinder wall and is scraped into the oil sump courtesy of the pistons leading to a faster blackening of the oil.
The particles are so tiny that they are able to escape the oil filter irrespective of how new or good the filter is.
Every vehicle, provided it has been run in, has some carbon build-up in the engine with the amount increasing with the number of kilometres on the clock.

So when your mechanic changes the oil in your car, the golden amber liquid is quickly darkened by the residual oil and carbon build-up in the engine. The high soot production, part of the everyday running of the car, takes care of the rest and within a few days that lovely new oil can look rather dirty.
That change in colour is a sign that the oil is actually doing its job.