If the oil is dark, it definitely needs to be changed, regardless of driven mileage. In fact, cleaning additive dissolved products of incomplete combustion, keeping the engine clean and protecting it from dirt, that’s why oil darkens.
It is necessary to adhere to the one interval of changing the engine oil for all modes of transport. If for cars of different brands oil change periods may be approximately the same, the frequency for commercial vehicles should be calculated taking into account the mode of operation.
Regularly changing of the oil is not necessarily. In fact, if you skip only one shift oils, engine deposits are formed much stronger, leading to deterioration of the engine lubricating and slower oil circuit, which may cause increased engine wear and its possible damage.
The oil is more expensive, so it is better. Even the most expensive oil can be harmful if it does not correspond to the vehicle manufacturer’s requirements, so that the main criterion in the selection of motor oil should not be the price, but specifies in the operational instructions.