Why does engine oil need to be changed? An interesting question, no doubt. It is also a logical question. Theoretically, MarineDiesel engines are sealed with gaskets and a multitude of filters. Engine oil should never get “dirty”. Yet, it does get dirty, and eventually turn to sludge. Why is that?
The short answer is due to chemical change.
Oil breaks down over time. It changes chemically as it sits in the engine. As the oil breaks down, it loses its’ lubricating and cooling properties, (the reason it is needed in the first place), and can perform those functions less efficiently. That is why your MarineDiesel manual will require a change of engine oil at least annually, or more often, depending on use.
The other reason is that, despite the engine being “sealed”, minute amounts of microscopic particles will enter the engine from the air, water, or through wear and tear of the engine itself. Of course, lack of maintenance exacerbates this process, and can quickly turn the oil into sludge as the engine runs. Gaskets wear out. Filters become clogged. Extreme climates add to the problem. These microscopic particles can be difficult to detect without specialized equipment. Many MarineDiesel distributors have the equipment and training that is necessary for this testing, and you should contact them for assistance with ongoing problems related to oil.
MarineDiesel recommends the use of synthetic oil in our engines. The paragraphs below are from a service bulletin we issued when we changed from natural to synthetic oil. The type of oil is extremely important in keeping your engine operating trouble free.
Marinediesel is changing the oil specification for all VGT engines from 15W/40 API:CJ-4 (mineral oil) to FULLY SYNTHETIC 15W/40 API: CJ-4.
If a 15W-40 quality is difficult to find on the operators location 10W-40 or 5W-40 oil in the same quality range is perfectly suitable
Those oils (10-40/5-40) are also strongly recommended for cooler climates, but can without problems be used globally
Background for change:
It has been seen in a few of our in-house durability tests where the engines are run at tropical scenarios and at high load for prolonged time that there is a potential for deposit or sludge build-up in the turbo housing. The turbo is cooled both by coolant and the oil itself, the oil temperature is well within limits of what a mineral oil can handle but the wall of the turbo can exceed these temperature limits under severe conditions and cause some deposit building up on this wall. These deposits could over time cause disturbances with the hydraulic actuation of the VGT vane system in the turbo.
Synthetic oil will not cause deposit build-up even under severe conditions and we strongly recommend that all VGT engines are serviced with FULLY SYNTHETIC 15W/40 CJ-4 oil or similar quality/weight as mentioned above.