Every engine will have all the oil drip back into the sump overnight. When you fire it up in the morning it take about 15-45 seconds to get the oil circulated in the engine. Diesel's sitting cold have to work harder since their ignition is solely based on cylinder temperatures. So when a diesel is started it takes time to get the temperature up and in that time engine knock occurs.
If the oil pump was bad, oil check light would come on in intervals during a drive and the car would overheat. Diesels rely heavily on oil cooling as diesels run twice operating temperatures and compression than gasoline engines.
In most of Pakistan there is no need to run a low viscosity oil unless the car is intended to run in sub-zero atmospheric temperatures.
Low viscosity oil's tend to shear quickly in a diesel engine when its under load.