I had a lengthy discussion with the service manager at the Nissan dealership where I bought my '14 Altima, as well as where we take it for service. I negotiated oil changes into the purchase when I bought it new, and so we take it there for every oil change @ 5,000 mile intervals. I was always of the understanding they were using the 0w20 oil that was spec'd for the engine by Nissan, but I found out after looking at the work invoice after we took it there about a year ago for one, that they were using a 5w30, and a non "Full-syn" one at that. During our discussion, I pointed out within the manual provided by Nissan where it said to use 0w20, and he ended up taking a while and finding some TSB that he had received a couple years back where apparently Nissan says that 5w30 is acceptable.
We discussed the whole warranty issue, and he pointed out within the document where it basically said it would not void coverage, etc.. and I reminded him of Magnuson-Moss and the fact I had a 7 year / 100,000 mile warranty on the car. Since it's the dealer doing it, and I keep all the paperwork they give us after each service, I can live with it.
Also, I live in Phoenix, and I figure the thicker oil may potentially work better in the hot summers. Heck, I frequently run 5w30 in my Fusion, which calls for 5w20.