You have correctly analysed the reason for your problem: Repeated short runs from cold starts. The DPF starts to regenerate passively after 20 miles of driving and burns off the soot in the DPF from its original cold start. It also regenerates actively (by introducing extra diesel to start a fire in the DPF) every 250 miles or so. But if the DPF is too clogged with soot, the active regeneration cannot take place and the extra fuel dribbles down the bores contaminating your lube oil and raising its level. Decent superfuel such as Shell V-Power Nitro Plus helps (I ran a Mazda 6 Skyactiv on it for 14,000 miles, no trouble). Stuff like 'Cataclean' might help but I can't recommend it because I've had no positive reports. Good to change the engine oil anyway every year or every 10,000 miles whichever comes first.