Oil Pressure Gauge

I have a question about the oil pressure gauge in my truck:
When I first start driving, the needle goes up to about 70. (The highest is 80, I think, midrange being 40) After driving a while, it starts to sink back down, eventually sitting around 40. As I drive, it goes up a little, and each time I put my clutch in, it drops quickly, often below 40.
Is this behavior normal?
I have only noticed it since the problem with my truck started. When the problems I have been having occur, the oil pressure needle is always at least around 30, and when it starts reving itself at intersections, the oil pressure gauge needle goes up to about 55 or 60 and back down. (while the car is not in motion)

At this point I am investigating everything that may be even slightly involved with my problem.

Thanks for any help/info

I have a 91 S-10 Blazer, 4.3L. I'm not sure of your year, but as far as I know, some of these trucks are known for faulty oil pressure gauges.

http://www.autosafety.org/article.php?did=51&scid=46

Says that:
"Oil pressure gauge flutters &/or reads high 1990-91 Blazer"

My gauge flutters, drops to the bottom and shoots up right to the top randomly. If you're worried about it, try putting in a mechanical gauge.

you are experiencing normal oil pressure fluctuations. you have to remember when the engine rpm goes up the oil pump rpm goes up with it creating more oil flow which creates more oil pressure. when the engine is cold, the oil is cold and it is hard to flow (viscosity) creating that high pressure at cold startup. as the oil temp rises the flow (viscosity) is increased (easier to flow)lowering the oil pressure.