it is now specced for the 20 weight, i have the o8 Dakota with the 3.7l. probably i will run a 5w25 in it though. if i plan on a super long OCI then i will use a straight synthetic, otherwise i mix it 50:50 group3:group2+
In in 2007 they started using 5W20 oil for the 3.7L engine. I had checked today and Mobil actually suggests a 10W30 or 0W30 and makes no mention of 5W30 in their "Ultimate Oil" line up for that engine in 2005.
my '08 3.7L did not impress with M1. M1 5-20 was great for about 1800 miles but then started running like the FF at 3,000 miles. Vibration and valve tick. Dumped it around 4,500 and now PP 5-20 in there. It feels better, less of a "rumble" feeling and less noise, but only 1,000 miles on it so far.
I personally feel that my particular engine would be better-suited for a x-30 weight, but the manual is a little vague as to what will and will not be suitable for warranty purposes. It does call out the MS-6395 criteria as needed for warranty, and I believe that some x-30s carry the ms-6395.