If the car is used for short journeys or the DPF is blocked then the mechanic is probably correct. The job of the DPF is to filter and store soot from the diesel engine before burning it off during a regeneration cycle. Passive regeneration takes place while driving, using the heat of the exhaust. If a passive regen isn't possible then the DPF will attempt an 'active' regeneration by injecting extra fuel in order to increase the temperature of the exhaust gases and burn off the soot. If the filter is blocked or the engine switched off mid-regen then the fuel may end up in the sump, contaminating the oil and raising its level. A diesel car needs at least 15-miles per journey to reach its optimum operating temperature and cycle the DPF. Failure to do this will lead to costly problems. This means you need to increase your mileage or change your car for something that's more suitable for your needs - like a petrol or EV.