I think the 10W30 (or at least 5W30) is better, in the summer especially, than the 5W20 that Chyrsler wants in it probably to meet CAFE and MPG. That stuff is so thin I have to wonder if the engine internals have any oil on them after sitting half the night. I would not run it in the 3.8. I am not a believer in running 20 weight oil in 90 degree weather. Hell I've had 4-stroke motorcycles that redlined at 13.5k did not use 20 weight oil.

I would not be surprised if the 10W30 lowered your milage .5 mpg. You will also lose a slight bit of power, but it should not be real noticable. I simply do not believe it will harm it.

But if Wally World overfilled it, that might cause problems. Might want to check.

I had startup clatter and valve train noise in my hemi (1000 miles on the motor) on the 0W40 it came factory with. Thats metal contact, no way around it. Found a Chrysler TSB, it instructed the dealers to use 15W50 Mobil 1 and change the fill cap sticker for customers complaining about noise on the 6.1. I run that in the summer and 10W40 in the winter. It starts and runs quiet and can still rev just fine. Had no noticable affect on mpg.

Just another opinion.