I hate the way GM is getting away with requiring Dexos licensed oils. To me that is like saying you have to use GM parts only which is illegal. I wish they would close this loop hole in the Magnuson-Moss Act and not allow a car mfg to require an oil be licensed to their own spec( I don't mind API as that is 3rd party ). Most car mfgs have an oil spec and just recommend you use an oil that meets it whereas GM seems to be requiring actual certification/licensing now. GM manuals used to say if you can't find Dexos use API w/ Starburst. That is what I would have done with a GM. I looked at your manual though and that is gone now. Now it just says use Dexos or else.
Nothing super special about Dexos either. Just a way for GM to grab extra money.
With all that said while under warranty you should only use a licensed oil. Use the correct weight too which is 5W30. Change it on time as well. I think the Service Writers you spoke with who recommended 6/5000 and to ignore the OLM are doing so as the engine is a DI model and they are very hard on oil. GM had to recalibrate a lot of OLM's in vehicles that 1st used these engines as they were set for too long. Maybe they are just suggesting to be safe go with 6/5000. To me that is reasonable.
I do not agree with any of them saying not to use full synthetic. A full synthetic will stand up to the abuse in a DI engine far better than any blend will. Blend is better than conventional and full synthetic is better than blend.
If money is a concern head to Wal-Mart, or the local parts store, and grab whatever the cheapest full synthetic on sale is, that has the Dexos mark, and use that. I would not run the $8 a qt GM stuff. That is just rebottled Mobil oil anyway. Mobil used to do GM's oil anyway and I have not heard that has changed. NO sense paying $2-$3+ more for Mobil oil in a GM bottle IMO.
Another option for you if you don't mind ordering is Kendall oil. They make fantastic oil and it can be ordered online very affordably( extremely hard to find locally ). The GT1 full synthetic 5W30 is Dexos licensed/certified and meets your warranty needs. Would be a great oil IMO to use if money is a factor. Youc an order it from PSC for great prices even shipped. The only thing is to get the ebst shipping deal order 2 cases at a time( not more and not less - best deal is always 2 cases ). IF you have a UPS Shipping account many times the shipping will be free from them too as a bonus deal( not free unless it says so at checkout ). To have the 5W30 shipped to my zip it would be...
$45.68 X 2 cases = $91.36 + $20.74( shipping to my zip - varies ) comes out to $112.10 for 2 cases( 24 qts ). $112.10 / 24 qts = $4.67 p/qt. That is even less than the 5qt jugs sold at Wal-Mart as a rule. Kendall oil is excellent as said above and would be a great choice. PSC always has it and for a great price so all you need do is order ahead.
2014 Impala is a nice car btw. Good luck with it. Just do your maintenance on time and with the correct fluids and parts and it should serve you well for a long time.