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Fuel accumulating in the engine oil, thinning it until it can no longer maintain pressure or lubricate properly. On a modern diesel this is often a direct consequence of the particulate filter, which is what makes it worth naming: regeneration injects extra fuel late in the cycle to raise exhaust temperature, and some of it runs past the rings into the sump. A vehicle whose journeys are too short to complete a regeneration repeats the attempt and dilutes a little more each time.

The oil level rising is the diagnostic sign, and it is counter-intuitive enough that people assume a misreading. Oil that is above the full mark on a diesel that has not been topped up is diluted until proven otherwise, and it will smell of fuel. Modern engines model this and will demand a service before the level becomes dangerous — a service demand at an unexpectedly low mileage is that model working.

Petrol engines reach it another way, through a leaking injector, prolonged over-fuelling or repeated cold starts that never warm the oil enough to evaporate what condensed. The consequence is the same and it is serious: diluted oil loses viscosity at temperature, so pressure falls, hydraulic actuators respond poorly, and bearing wear accelerates. Change the oil and find the cause — an oil change alone resets the symptom and not the mechanism.