Former mechanic and 3800 owner here. That's a normal-looking O2 sensor in my opinion, and you said it was cycling normally. I suspect your fuel efficiency woes are elsewhere. Is the engine low in power? Could be a plugged cat. As long as you're checking the brakes with a temp gun, might as well shoot the cat too. And give it a couple of love taps to be sure the catalyst isn't rattling around inside. It should temp at least a couple hundred degrees higher than the downpipe. If not, well, it's up to you what to do about that....