you bet it is - the oil never gets hot enough to clean itself and then makes sludge and sticks it everywhere on the engine like the hooligans of every political party in Pakistan stick posters to walls.
No interval, you should replace that car - diesel cars are usually bought when the driver has to drive at least 50 kms a day, it takes a good 10 or 15 minutes drive to warm up my 6 cyl diesel covering about 5 miles. Using it for that short distance is strange, it must be terribly coked out too.
when I drive (btw I drive 165 kms a day) and if I have to stop shortly by a pump or get something to eat I leave the engine running, it keeps the engine nice n warm.
And I do spin it to 5500 rpm usually to keep it clean. (btw dont try that, I do that because my engine revs like a petrol engine)