Even if the gauge looks new.. spend the 9 bucks for piece of mind. Luckily that 8n is ALL good old sae.. and you shoul find the adapters needed for it in the sunpro gauge pack.
While I agree with the other poster that a sudden drop in pressure can be a clogged line.. a rise in pressure is not a usual symptom.. however.. when you do change the gauge.. leave the fitting a tad loose and throw a rag over it.. crank the engine over or start it at low idle till you get the air bled out of it... Air bubbles up at the gauge can cause weird readings too.
Once you repalce the gauge, and it is still high. Kill the engine and go take a look at that plunger / spring. While I think the FO-4 may give the inch pounds and length of the spring.. I think an old one is gonna be a gamble on getting it adjusted.. however a new spring is cheap.. still.. if you feel up to it.. you might try bending or shortening it a loop or so.. If you start it up and have no oil pressure, you have lost prime, and / or the spring is too loose and letting all the oil bypass. Stretch the spring back out or replace to account for the lowered oil pressure. If it did loose its prime, get some heavy oil.. egen gear oil.. anything that is thick.. even stp motor addative.. pour it down the relief hole till it is full.. slap the spring and plunger and cap back on super fast, and then get that engine cranking over to get the pump primed. As you may have guessed, the oil relief setup is above the oil pump, and dribbling oil into it fills the gearing. Loss of prime while dinking with the plunger / spring is not common.. but not unheard of either. Let me see if I have some pics for you.
Soundguy