It's possible that when the car does an active regeneration, that some of the fuel is not vaporized correctly and ends up in the crankcase, diluting the oil. Diesel fuel will slowly evaporate but I'm not sure if it would create as much as 3-5mm. This is just a guess that is what's happening on your car because you say it's related to the regen.
Look through the FAQ section for more on the DPF.
Look through the FAQ section for more on the DPF.