High Oil Pressure

I have a 99 jeep wrangler sport 6 cyl, auto tran, with about 96,000 miles on it. Noticed engine oil prissure all the way over to 80 and not sure why. I got the tran and enging oil changed 6,000 miles ago and noticed it then. Well got it done on 7 july 07 then went for a couple weeks but didn't pay attention to it. End of month went on a 350 mile drive then noticed it. I checked and they used 5w30 enstead of what the book recommends (10w30) The weather has been in the 90's all month. Could this be the problem. When I stop at a light it drops down to about 70. Just got the Oil changed(15Aug 07) and same oil was used and they put a new filter on. I didn't know pay attention to the type of engine oil they used till today the 17th of Aug. Is it something other than the type of oil?