I don't understand how just reducing the oil level is a satisfactory solution to the problem. I agree with
@DXX, the cause is likely to be diesel entering the sump due to failed regenerations. As others have said, oil analysis would prove or disprove this.
So reducing the oil level just means that people are driving around quite happily with a sump containing a mixture of oil and diesel. (or worse, coolant!)
IMHO if the oil level needs reducing then the oil should be drained and it should be refilled with the correct amount of new, uncontaminated oil. (and a new filter should be fitted).
To just keep removing some excess oil is kicking the can down the road and might end up being a very expensive mistake.
Just my non expert opinion.