I used to own a 1.6 turbodiesel VW Golf in Pakistan, that car used to return between 22-24 km/lit easy, engine oil used was usually caltex Delo 400 15W40, changed at every 5000 kms along with its oil filter.
Even at 5000 kms the oil still was clean by the tile test. The running condition of the engine was that it had the oriignal 89 degree thermostat in place, filled with coolant, original Victor Reinz Headgasket with new bolts. The original crazy powerful cooling fan setup. The gauge was always at 60% of the sweep no matter if I was in Karachi in the summer or Quetta in the winter.
If you fulfil all operational requirements of the engine it will run like its supposed to, So if you are running your engine with no thermostat and complaining of low power, oil sooting up and expensive running you yourself are to blame.
I still drive a diesel here in the US, its a mercedes benz E300D, 3.0 6 cyl DOHC 24V diesel and I get about 13 km/lit from it. oil changes every 3500 miles along with filter and 85 degree thermostat in place cooled by Valvoline Zerex G-05 (MB coolant), the engine only makes its smooth quiet power when its hot, running it cold literally feels harsh.
Now when I read that a toyota 2C in a corolla returns 10 km/lit I find it hard to believe unless the engine smokes black all day long.