You should always be using at least 5W-30 oil. These new engines might be able to use thicker oil, but the return holes in the heads are not big like they used to be in the old days. With sludge problems these motors CAN have; and other complications, you can starve the motor for oil with it all up top. I have seen this happen personally, with an intrepid, and a 4.0 Jeep Cherokee my pap used to own. He caught it in time. amd all was fine, sludhe clogging the return holes on one bank
Personally; I used 10W-30 in V6 my Avenger all the time, never had a problem... I used 10W40 in the summer, still never had issues, was just fine, though people REALLY say not to do that. I am using 10W30 Synthetic in the Liberty now...