I worked in 5 different dealerships while trying to make a career change. Depending on just what the claim is and how much the repair will cost determines how much of a hassle it is going to be. Dealership sales volume comes into play too in cases of a big claim. It seems the bigger dealerships sometimes have more pull, that was the case with a big Honda Dealership I worked at. The MFG reps can really bust balls if they want to.
Now back on topic, a 20 grade oil will work just fine in the 09 Jeep, and not cause warranty issues.
I based the above statement on real world experiences working in auto sales, and knowing the service managers very well. What people fail to realize that if a major claim occurs and the car is down while waiting for a resolution it is that person who is not driving their car. Business goes on at the dealership uninterrupted. I've seen cars parked for a month or more waiting for issues to be resolved, the customers were not happy. The staff still rides home and their life continues w/o issue, it's the customer who is SOL.
All I'm saying is why chance it? Or if you feel you really have to run a 30Wt oil in a car spec'd for a 20Wt, make sure you keep 2 sets of books, and hope you never have a problem, or if you do you're a lucky person.