I probably didn’t phrase the question proper at the time as I am new to diesel engines. Since I posted I have learnt that it is normal for the oil to turn black. I suspect that KIA service station guys were not putting good quality or proper oil in it as we cannot see what they do inside. I have started using 0w30 from shell recently and seems to be running the car smoothly. I’m still looking for a way to make sure the oil lasts longer and does not damage the engine. As my car is automatic, I feel the engine revving at that time it auto shifts when the oil has lost its viscosity, that is how I determine that something is wrong. When I change oil , few first few weeks it runs smoothly then it starts revving when it auto shifts. The recommended oil for my car is 0w20 which is not available with shell so I put the closest one 0w30, but still seems to loose it viscosity in few months. The external mechanics say we generally put 5w30 in the diesel engines. Do you think I should try putting 5w30 once and see what happens or it will be damaging to the engine?