To find the correct oil, you MUST check your owner's manual. This manual will specify the recommended viscosity (like 15W-40, 5W-40, etc.) and the required API certification (like CJ-4, CK-4, etc.). Using the wrong oil can severely damage your engine.
Generally, modern Cummins engines in Dodge Rams often call for oils meeting the CJ-4 or CK-4 specifications. However, older engines may require different specifications. The viscosity will also depend on your operating climate; thicker oil (higher second number in the viscosity rating) is often preferred in hotter climates.
Don't rely on generic advice online. Consult your owner's manual for the definitive answer. If you've lost your manual, you can usually find a digital copy on the Ram Trucks website or through a quick online search using your truck's VIN.
