i have a 1990 5.o
when i turn on the car my oil pressure is at the middle of the gause and as i drive the gause goes down not all the way down but it goes down
Cold (ambient) oil will always have a higher pressure than oil at operating temps (200F). So, what you're seeing is normal. That being said, a "real" oil pressure gauge is highly recommended over the stock one (which have a reputation for being very inaccurate).
As long as you hear no abnormal noises from the engine, you most likely have no problem. If you want to verify this, buy a $20 mechanical oil pressure gauge at the local auto parts store. Install it in a temporary manner. Start engine, take a cold idle reading. Rev engine to 2500 or so, take another cold reading. Repeat the same two steps after a 15- 20 minute drive. Once you've taken the reading, re-install the stock sending unit.
Cold idle should be 35-60psi. Hot idle should be 20-35psi. Note: I've listed a fairly large range as I don't know the history of your engine or type/viscosity oil being used.
Yeah the stock oil presure gauges are junk. If your oil pressure is dropping when you are increasing RPM's, then you have a bad sending unit or a big problem. Oil sending unit is easy to get at, it's on the front drivers side of the engine.
I replaced mine with a mechanical unit and have a copper line run up to the dash. I also wired in a sensor that sets off a warning light when the oil pressure drops.