Diesel engines have very specific lubrication requirements, and some diesel engines can place extreme demands on oil, so it's important to make sure that the oil chosen for your diesel engine is up to the job. There's also a number of other things to consider when thinking about oil in diesel vehicles.
It's important that the right oil is used in your diesel and that you don't mix brands or formulations. This means that you should never swap between or mix mineral oil and synthetic oil blends.
If a vehicle manufacturer specifies that only synthetic oil must be used, there is usually a very good reason for it. For instance, on some VAG group vehicles (VW, Audi, etc.) the high-pressure injection pump is driven by the engine via a sort of camshaft that requires a very high level of lubrication not to fail. No mineral oil formulation, no matter how advanced can provide the required level of protection, which means that using mineral oil where synthetic is required invariably leads to major failures on these VAG group vehicles.
While it is possible (but not recommended) to extend oil change intervals on most petrol engines, the lubrication demands of diesel engines are such that the oil in diesel engines degrades much faster and more completely than the oil in most petrol engines.
What this means in practice is that if you exceed prescribed oil and oil filter change intervals, you begin to run the risk of running your engine on oil with a severely reduced capacity to provide adequate lubrication and protection against corrosion, mechanical wear, and sludge formation, which could lead to an expensive breakdown.
One of the dangers of over filling a diesel engine with oil is that the additional oil may come into contact with the spinning crankshaft, which will whip the oil either into foam, or sometimes, into a fine mist. Now, since the purpose of the PCV (Positive Crankcase Ventilation) system is to ventilate the crankcase by extracting the gases that form as a result of normal engine operation, some of the oil mist or foam will also be extracted and fed into the air intake system to be combusted along with the regular air/fuel mixture. While this might not affect combustion overly much, the additional hydrocarbon load will almost certainly have very serious negative effects on both the catalytic converter and the DPF (Diesel Particulate Filter), both of which are required to be in perfect working order to control exhaust emissions effectively.
If you have any questions about the right oil for your vehicle, contact your local Repco Authorised Service centre