The day after your car was serviced you have demonstrated your lack of trust in a mechanic and decided to check your engine oil, just in case you have parted with some cash just for the change of a small sticker on your windscreen. Given that you have already entertained this idea in your head, it’s no real surprise that your looking for even a remote slither of evidence to back up your preconceived idea that mechanics are all ex inmates practicing hard learnt tricks of the trade from within the clink. The removal of the dipstick instantly confirms your suspicions, the oil is not the golden glow you know it to be from the commercials you see aired whilst Bathurst is on. Your mechanic must have ripped you off.
Unfortunately you are making assumptions and jumping to conclusions with the very limited knowledge you have and it’s a very small piece of the puzzle. First its important to clear up some of the science behind what is actually happening inside your engine to cause oil to go from that lovely golden colour to the black of the devils urine.
Engine oil carries out three vital functions whilst its in its engine. The first and the most widely known fiction of the oil is to lubricate. The oil is used to reduce friction between moving metal parts within the engine and prevent them from completely locking up, resulting in catastrophic engine failure.
The second function of the engine oil is cooling. The oil is used to transfer heat from those metal engine components and absorb that heat take it to the engine sump where the air flowing under the engine sump is intended to cool it.
The third task of the engine oil is to clean. Engine oils have detergent in them, in the same way detergent is added to your dirty washing and used to clean it, your engine oil does the same thing. Now imagine your clothing is soaked in carbon deposits, the waste water coming out of your washing machine will be blacker then the ace of spades (assuming your detergent is any good) This is exactly what is happening inside your engine. The carbon is getting cleaned out of your engine and thus turns the golden oil black.
This still does not answer the question as to why new oil has turned black almost immediately. The answer here depends upon the kilometres your vehicle has done and how much carbon build up is inside your engine. New vehicles when the oil change is carried out will retain the golden colour for serval hundred kilometres as there is minimal carbon build up inside it. A vehicle with more kilometres on the clock will have large amounts of carbon through the engine, this is not a bad thing but rather is just a byproduct of owning a vehicle and driving it.
So your new engine oil in a car with more than forty thousand kilometres is immediately added to a high carbon environment and it instantly does its job. The truth is your new oil started turning black the day it went into your car.