You didn't say if this went from normal to overheating gradually or all at once. And I'm assuming that you would have mentioned if it was running bad with visible white smoke. So it sounds like the t-stat could be the culprit. But regardless of what it is, you need to get it fixed asap because these engines often blow head gaskets and/or warp heads on big overheats.
On the oil, do youself a favor and read some information about current oil technology. You can find lots of stuff on this and they all say the same thing. 5K mile OCI is consertative these days - many auto makers are using 7.5K. It took me a while to come around to this - something about an old dog being slow to learn a new trick