Ask the Mechanic: Is there a right and wrong way to change oil?

Is an oil change as simple as draining old oil out and pouring new oil in, or is it more complicated than that and, therefore, possible to do well or badly? Other than checking the level on the dipstick after service, is there anything else car owners should monitor? Also, is engine oil still okay after it turns black?

Peter

Hello Peter, there are definitely right and wrong ways to change the oil in an engine. Assuming the new oil is of a good quality and a suitable grade, filled to the correct level, the most likely mistake will be how the old oil is drained out.

Oil should be drained as soon as possible after the engine has been running at normal temperature, so the oil is hot and runny and all the “dirt” in it is still churned up. Also, the filler cap must be removed before the drain plug is loosened. And the vehicle should be on a flat surface.

 All of that is necessary to ensure the oil flows out with maximum speed, carrying all the dirt with it and leaving as little as possible of the old oil clinging to the walls and components of the engine, to minimise residual sediments.

 Doing this when the car is flat is important because the sump is shaped to ensure everything drains out when it is horizontal. If the car is tilted, the drain plug hole will be above parts of the sump and ponds of old oil will be left behind.

Oil changing colour

 The black colour of old oil is mostly soot generated by combustion in the engine, which accumulates over time. In new petrol engines, the quantities are minute, and even though there are millions of combustion strokes between oil changes, the oil can remain clear and golden. As the engine wears, contact with the soot in the combustion chamber and elsewhere increases, the level of soot in combustion increases, residual dirt accumulates, and the oil can become black well before an oil change is due.

 This source of soot is microscopically fine and not hard, and, therefore, not significantly abrasive, so the oil can still do its lubricating job adequately (though not perfectly) until the due time for change, as long as there is not too much other detritus (shavings of metal from engine wear or dust particles from the air ingested for combustion) suspended in it.

 If the oil gets so dirty with soot and other substances (more prevalent in diesel engines) that it becomes less thin and slippery and more thick and sludgy, it should be changed without delay, irrespective of the scheduled service interval. It is the consistency and smoothness more than the colour of the oil that matters. If you take a sample of oil from the dipstick and rub it between your thumb and forefinger, and there is any sense of grit or stickiness, change it.

 If the oil is not drained properly when being changed, some ‘sooty’ oil will be left behind to mix with the new almost immediately. That is why the correct draining process is important.

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