Ahhh, first, the manufacturers switched to 5w-20 mainly because of CAFE .(corporate average fuel economy). Don't let them tell you anything else. Maybe they claim some other benefits, but CAFE is the main reason for the switch.
I live in a hot climate (south Florida) so I use 5w-30 RP.
They can't void your warranty for using something other than 5w-20 as long as it is API certified. The dealer can try to tell you different, but have him show you where it says that in the Magnuson-Moss Act.