I'll try to document my adventure in removal of a stuck oil pressure relief valve for others who may have a similar problem. Many of you will not have to go to the extremes I did and only need to try some of them. Maybe this will be a source to consider should you ever encounter similar problems.
Excessive oil pressure can be noted by a pegged oil gauge. If your engine is equipped with an oil filter, oil may be forced past the oil filter gasket. Spin on filters may blow the gasket out of the oil filter no mater how much you tighten the filter. The high oil pressure reading may be only at startup and may be followed by low oil pressure afterward. Oil pressure relief valves can stick both ways, sticking closed at startup and open after warm up and exhibit low oil pressure.
An initial attempt at determining how stuck your relief valve is can give you some clue as to how far you will have to go to fix your problem. A newly installed relief valve that is well lubricated should easily slip in and out of its bore by inserting the tip of you pinky finger and pulling it out. In my case I started with it completely stuck and rusted fast. The best advice to start with is to attempt to lodge a 1/4 npt pipe tap into the end of the relief valve piston. the tip of the pipe tap may need to be ground down to facilitate a tight fit without bottoming out in the end of the piston. Use an open end or adjustable wrench to turn the pipe tap while taping the square end of the tap. The metal the piston is constructed of is case hardened and the threads of the tap will have difficulty biting into the metal. With a flash light, look past the tap to see if the piston is turning in the bore. If not, spray with PB Blaster and continue to work it alternating spraying and tapping the tap while turning it with a wrench.
At some point if you are failing to make headway, it may be necessary to apply heat. The ability to concentrate as much heat to the inside of the relief valve piston will reap the most reward. An Acetylene torch will work best if you are fortunate enough to have one. Mapp gas can also work but you will not be able to concentrate the flame as much. Alternating the method mentioned in the last paragraph and heating with the torch. I found for my case which was one of the more extreme I had to heat the piston to red and allow it to cool before using the pipe tap and tapping. It may take a while but several heatings and coolings will gradually help to loosen the stuck valve. Do not heat the metal so much that you are seeing the metal melt and flow. Heat it slowly to red, and let it cool.
The heating of the metal will serve two purposes. First the heating and contracting of the metal affects the hardened piston and the cast block differently. The piston will expand in the bore of the cast block and spread the bore slightly. as the metal cools the casting will tend to retain it shape while the hardened metal will more/less return to its original shape. The piston will also lose its hardness or temper making it easier for the pipe tap to get a bite.
I'm my case the piston lost enough temper and the pipe tap got enough bite that the piston could then turn in its bore. I the used some emery cloth to shine up what was visible of the bore. I chucked the pipe tap in a drill and began working the pipe tap/piston back and forth in bore and gradually was able to pull the piston out. I cleaned up the bore best that I could so that the new piston was able to move freely in the bore. There were no easy remedy's in my case. Persistence and elbow grease, an acetylene wrench, accompanied with a little percussive persuasion was what it took to get the job done.