What is the best recommended oil

Welcome to 2CarPros.

First, here is what Chrysler recommends based on temperature:

Engine Oil (6.7L HO Diesel Engines)

Ambient temperatures below 0 deg F (-18 deg C)
Engine Oil. SAE 5W-40
- SYNTHETIC engine oil that meets Chrysler Materials Standard MS-10902 and the API CJ-4 engine oil category is required.

Ambient temperatures above 0 deg F (-18 deg C)
Engine Oil. SAE 15W-40
- Engine oil that meets Chrysler Materials Standard MS-10902 and the API CJ-4 engine oil category is required.

Use only Low Ash Diesel Engine Oil meeting standard MIL-2104C or API Classification CJ-4/SM (5.9L), or CJ-4 (6.7L), or higher that meets Cummins CES 20081 Standards.

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As far as conventional or synthetic, you can use either. If the vehicle is worked really hard, I would personally use synthetic because it has less chances of break down. However, under normal use, conventional is fine as long as it meets the criteria listed above. If you are concerned, use a semi-synthetic. That way you get the best of both worlds.

Let me know if you have other questions.

Take care,
Joe