Well theres a bunch of factors as well. GM's "spec" i believe is 6psi per 1k rpm MINIMUM. You say everything was normal until you changed oil 800 miles ago, what type of oil was it? Humans make mistakes, im sure that it could also be a heavier weight that your dealer or whoever did it may have put the wrong stuff in. Its always possible.
Oil pressure is NOT created by the oil pump, that creates flow and volume, they do not create pressure. what creates the pressure is the gaps between your metal parts, your connecting rod bearings and main bearings create the oil pressure. Im sure your dealer will say its fine just because that what dealers usually do, if they find a problem and fix it then great, if not, id start with another oil change. Do the oil change yourself if possible to eliminate some variables. I hope the dealer finds something out or fixes the issue! If not come back to the forums and im sure a bunch of us can give you some ideas!