Assuming it is diesel engine then you MUST use oil that is for a diesel engine.
Shell Rotella or Mobil Delvac are examples of “proper” oils.
I won’t start an oil war by making any recommendations but I do use the Rotella just because it is convenient, available and suitable.
The oil in a diesel engine will normally be black after a few hours. Newe models that have emissions control supposedly don’t turn black (as fast).