Diesel dilution is also a common cause of engine failure as its effect on oil is that it causes viscosity to drop dramatically, the more diesel, the lower the viscosity.
The lower the viscosity, the weaker the film strength. Diesel readings of around 0.5% can often be reduced by controlling excessive idling times, however any reading over 1% should be acted on as this will cause a significant viscosity drop by around 20% or more.
The higher the diesel dilution, the lower the flash point which means that a crank case can ignite if the sump temperature reaches say 130degC with 6% diesel dilution, which is the temperature at which that concentration of fuel vaporises and becomes explosive.
On the other hand, care needs to be taken when interpreting diesel levels, as often the diesel in certain forms of unburned fuel will cause excessive sludging in the oil. This can cause the viscosity to increase and the oil can thicken to the point where it becomes un-pumpable in the engine, causing catastrophic failure due to oil starvation.
New engines have been known to fail through this cause of inefficient combustion by over fuelling, which can also be indicated by excessive exhaust emissions.
Always check a positive diesel reading as the black smoke and high soot levels not only mean increased and excessive wear to an engine, but they also equate to money burning out the exhaust pipe. Soot particles are 4 times more abrasive than dust particles and are a major contributor to wear. They get larger as combustion efficiency decreases.
Never wash or flush equipment with diesel as any residue remaining will dilute the new oil reducing it's viscosity and destroying the oil's integrity.