Most people don't understand oil labels. Many falsely believe the W means Weight....it doesn't. Oil weight, or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).
Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). The "W" designation means Winter. Again, the temp ratings are for oil temps not ambient temps.
The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 210 degF. This is a fixed limit and all oils that end in 40 must achieve these limits. Once again the lower the number the thinner the oil, a 30 oil is thinner than a 40 oil at 210 degF etc. Your handbook will specify whether a 30, 40 or 50 etc is required. Dodge says 40. Again, Dodge specify's a 40 so that the Viscosity at 210F meets the engineers requirements. It does not mean a 50 performs better at higher temps. Big difference....
Dodge engineers want a thinner viscosity oil thus the 0-40 requirement.