I just purchased(accidentally) 5W-20 oil instead of the 5W-30 that the manual says I should use. I have very low miles on my xB and live in a very warm climate should I exchange it or just use it?

If its still unopened, Id exchange it for 5w30 myself, if you are running dino.

If its synthetic, exchange it for 0w30.

I'd just use it. Toyota now recommends using 5w-20 for 06 and up cars but there isn't any change in engine dynamics so you'd be fine using 5w-20.

A big however though, is that it's summer and if you're on the west coast suffering from this damned heat wave (although today and the rest of this week seems to be pretty fair now) then you might want to get your engine that extra protection.

There's a thread somewhere around here with that TSB about the oil. I think it said '06 and up will be using 5w 20 now. That and you're paying attention to the maintenance intervals on your vehicle so I'd say you're ahead of the average car owner as is.