Well, 5W-20 has become kind of an irrelevant grade. In fact, the very first Mobil 1 5W-20 was actually a 0W-20, however, way back when, SAE hadn't yet published the specifications for 0W, so Mobil just marketed it as 5W-20. Today, blending 5W-20 is either an additional hassle for blenders to make, or an opportunity to save a few pennies in materials, depending how you look at it. And some love having an extra product in their portfolio to take up more shelf space at Walmart. All in all, it's a pointless grade today, given how close it is to 0W-20.
Even when under warranty, you can use whatever grade you deem appropriate as long as it's within reason. No, I wouldn't advise anyone to jump on Mobil 1 15W-50 and put that in their engine, however, 0W-40 or 5W-40 can be used in just about anything that calls for 0W-20. Tolerances and oil passages will easily accommodate for that. If an engine fails, it's usually due to a design flaw, or lack of maintenance. Using a motor oil that will provide better wear protection will not cause it to fail. The manufacturer knows that, the dealer knows that. Just never get into it with a know-it-all service advisor or service manager that don't know anything about lubrication.
When the 3.6 Pentastar PUG was released and used in the 2016 Jeep GC for the first time, it called for 0W-20. In fact, it still calls for that. The various Mopar forum bogeymen out there will tell you not to use anything lese, but MS-6395 0W-20 oil in it. Well, the bearing tolerance ranges in that engine are nearly identical to the 5.7 and 6.4 HEMI engines. And yes, you can safely use a 0W-40 or 5W-40 in a 3.6 Pentastar PUG engine. Though I wouldn't recommend anything beyond that, because they made some of the oil passages relatively narrow.