If they could actually tell. In over 40 years of both personal and commercial vehicle ownership, some with engines more expensive than many folk's complete automobiles or pickups, with some warranty issues sporadically over those years and I have never had a dealer shop even ask what I was using for oil, let alone actually jump thru all the hoops to test it or make me jump thru hoops to provide documentation of what I did. Not even so much as a simple UOA, which could not tell if the oil was the correct oil or not. And the diligence I put into maintenance, if they did test the oil, it would show it being well within serviceable condition.
I use a 10w30 conventional year round in my 2006 Cadillac CTS 3.6 that calls for M1 5w30 by GM, and I have had fewer issues than most folks with that LY7 engine. Still a great car and planning on holding onto it for quite some time (partly due it being old enough now to qualify for the very bottom annual tag registration of $50 in my state). The wife loves it. I alternate on my 2015 Silverado 2500 6.0L L96. I typically run a 5w30 in the winter and a 10w30 in the summer. If I had one of those newer engines that called for 0w20 and stayed with 20 weight in it, I doubt I would use anything different than 5w20 all year round. On the dexos1 Gen 1 list were several 5w20 oils. Doesn't matter. the dealer could never tell if I would be using a 5w20 or a 0w20 anyway. Pick a name off the dexos list and tell them that is what you are using, if you say anything at all. I have never had to jump thru hoops of piles of documentation to prove anything. GM, Ford, Detroit, Cummins, etc. None have even cared. And I have never seen ample evidence of OEM's asking such things to convince me it is an issue. One or two anecdotal stories on the internet does not a policy practice make. More often than not, the owner said something that caused the dealer to question things. It is like talking to cops. Don't say anything more than you need to. And then, act simple like Forrest Gump when you do.
Now if I move to Canada or Alaska, then I might consider 0w20. But overall, I don't suffer from an enlarged paranoia gland that makes me waste a lot of time worrying about what the OEM wants. Many commercial and fleet users dump in the same oil they use for all their stuff and the OEM's don't ever fuss about that.