The DPF regenerates both passively and actively. Passively the soot in it gets partially burned off by the heat generated by the engine. But every 250 miles or so, active regeneration will cycle that involves injecting the engine with extra diesel to effectively create a fire in the DPF and burn off any accumulated soot. It's this cycle that can go wrong and dump diesel down the bores into the sump, contaminating the sump oil and eventually rising to a level where a compression ignition engine starts to run uncontrollably on its sump oil. If you set the instant consumption indicator you will see this happening by an increase in fuel consumption. Important not to switch the engine off while it is actively regenerating.