My girlfriend has a 1997 Pontiac Bonneville SSE (3.8L GM 3800-II, non-supercharged). It ran low on coolant a few weeks ago, and started to overheat. I refilled with coolant, but the next time I was in the car, I noticed that the oil pressure gauge (which, according to the shop manual, should read around 60 PSI) has been hanging around 80 PSI, often increasing to 100 or higher (gauge goes to 120 PSI) and sometimes dipping down to 40 or so.
The gauge movement seems somewhat random to me. It will stay around 80 for a while, then fairly quickly climb up to 100+, sometimes hang there for a while, sometimes just drop back down to 80. It also seems to bob up and down quite a bit.
I know I've heard that the oil pressure senders in these cars are known to go bad, but I've got less than 500 miles on this oil, and would rather not have to change again so soon, especially since I run Mobil1 fully synthetic in it.
Can anyone think of a mechanical cause (or anything other than a bad oil pressure sender) that would cause these erratic oil pressure changes?