F150 Oil pressure problem

I have a 1995 F150 4.9L 6 cylinder that I was driving to work when the oil pressure went to nothing. I have checked it and it seems when it is first started it has good pressure until it warms up and you rev up the engine and then it looses all the pressure. I have repl;aced the oil pump(what a pain that was!!!) and I still have notsolved the problem. The engine has 100,000 on it. Any suggestions would be greatly appriceated. Otherwise I was thinking of swapping out for an 8 cylinder?

You most likely do NOT have an oil pressure problem (if it runs and has oil in the pan).

Beginning in the late '80s ('88?) Ford stopped installing "real" gauges. The pressure "gauge" you have is nothing more than a switch activated idiot light with a needle instead of a light. The switches are closed above about 8-10 PSI, and open below that.

Find the switch (usually near the oil filter), ground the terminal to the block, turn the ign to "run" then check you gauge, it should read in the normal range. If so, it is most likely that the switch is bad. Before you spend money or time worrying about the engine, replace the oil switch.

Thanks MOD for the information. I found that the idiot guage is indeed working. It seems when I first start the truck it takes a minute for the pressure to come up. It then holds the pressure until the engine warms up. At that point it goes to zero when you rev it up and comes back when you let off the gas. It will do this for about ten minutes and then the oil pressure goes to zero and stays there. The engine then starts to tap as if it is getting no oil. If you run it longer the tapping gets louder. The tap is coming from the topside of the engine, like a valve tap, only louder. I had said earlier that I have replaced the oil pump and it seems it did no good. Could the main bearings need replaced and would they affect the oil pressure? What else could it be? Please can anyone help?

If the mains are excessivley worn, you will "leak" too much oil past them, and not have enough for other things (like lifters). This is NOT a typical problem with the "300-6", they will usually run nearly forever even with low oil pressures. I would get a REAL gauge, and see what you really have when you hear these noises. The oil pressure switches are notorious for being flacky and not reading properly as temperature changes in the oil.