You've described two problems my '92 is experiencing.
The Temp guage has been like that for a long time (> 4 years). Since this question was posed, and I had my scan tool (OTC 4000) hooked up last night, I made note of the Coolant Temperature reading. Scan tool reads around 180 (more than the 140-150 normally on guage). I haven't changed to a t-stat other than stock, but when I started using ER (Energy Release) in the oil, I noticed my car never ran quite as warm. In fact, it seems that the temp started reading low about the same time I started using the ER. If you don't have access to a scan tool, but you have the Automatic Air Conditioning option (C68) in your car, you can get the coolant temp from your A/C control head. Though I haven't verified the readings against my scan tool yet, I posted instructions in a thread at impalassforum.com a few days ago
http://impalassforum.com/noncgi/ulti...;f=32;t=001071
As far as the oil pressure guage, mine just started acting up 2 weeks ago. It seems like my problem is more intermittent than specifically going away when the car warms up. The guage pegs at times, but reads normal most of the time. Oh, and neither the scan tool nor your AC unit gives any oil pressure readings. And I just realized there appears to be no idiot lights in our cars with the guages. The light on my AC unit which indicates "Low Oil" only tells you if the oil level in the engine is low, but not if the pressure is low.