BadMoon has the answer. Per your owners manual:
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Selecting the Right Engine Oil (6.6L 8-Cylinder Engine)
Specification
Oils designated as API CJ-4 are required for the vehicle. The CJ-4 designation can appear either alone or in combination with other American Petroleum Institute (API) designations, such as API CJ-4/SL. These letters show API levels of quality.
American Petroleum Institute (API) symbol
This doughnut-shaped logo (symbol) is used on most oil containers to help you select the correct oil. It means that the oil has been certified by the American Petroleum Institute.
Look for this on the oil container, and use only those oils that display this logo.
Caution: Use only engine oils that have the designation CJ-4 for the diesel engine. Failure to use the recommended oil can damage the DPF and result in engine damage not covered by the vehicle warranty.
Viscosity Grade
Use SAE 15W-40 viscosity grade engine oil.
When it is very cold, below −18 °C (0 °F), use SAE 5W-40 to improve cold starting. These numbers on the oil container show its viscosity, or thickness.
When selecting an oil of the appropriate viscosity grade, always select an oil of the correct specification. See “Specification” earlier in this section.
Engine Oil Additives/Engine Oil Flushes — API
Do not add anything to the oil. The recommended oils with the API service symbol are all that is needed for good performance and engine protection.
Engine oil system flushes are not recommended and could cause engine damage not covered by the vehicle warranty.
You cannot go wrong with synthetic. My choice happens to be Royal Purple.