Oils that are formulated for use in diesel engines differ from oils that are formulated for use in petrol engines. Foremost among these differences is the relative proportions or concentrations of the additives that are added to the base oil.
The total volume of the additive package can be as high as 25% (or sometimes more) of the total volume of the oil. While this is true of almost all branded oil formulations, the problem is that no two oil manufacturers use the same additives, or use the same additives in the same proportions, and herein lays a major problem for the average owner of a diesel vehicle.
It is often tempting to see all engine oils as just engine oil, but this could have disastrous consequences if price alone determines which oil you put into your diesel engine. To understand this better, let's look at oil chemistry. Oil chemistry is a hugely complicated subject, but suffice to say that the two single biggest contributing factors to oil degradation are the effects of heat, and contact with oxygen. Following closely behind is the fact that some additives can cancel each other out when oil formulations are mixed. In translation, this means that when you mix oil formulations it could cause two different detergents to clash, with the result being that both detergents are effectively deactivated, and the engine could accumulate a huge load of sludge very quickly.
The same is true for almost all other additives; for instance, viscosity improvers can combine to thin out the oil too much, or combine to raise the oil’s viscosity to the point where it cannot be pumped around the engine.
Vehicle manufacturers invest significant amounts to develop engines that are reliable, efficient, and durable, provided that the oil you use complies with the manufacturer’s recommended oil grade/formulation/viscosity. Engine manufacturers also know that engine oil does not last forever, and “wears out” in the sense that some additives are depleted, while others get ground down as a result of normal engine operation, all of which adds up to making sure you service your diesel vehicle regularly and use a high quality oil that is suited to your car.