i would try another oil pressure gauge.
remember: no oil pressure is the fastest way to ruin a good engine, second fastest way to ruin a good engine is over heating.
maybe the valve that controls the oil pressure is stuck. You do not need to run the engine until you are sure you have oil pressure. If you do not know how to fix it, have someone else look at it before you ruin a good engine.
it does not take long to ruin the engine with no oil pressure.
i use shell rotella diesel oil 15w40 in my 1957 T0-35, this oil still has the zinc in it as oils did have back in the day.
old tractor holds good oil pressure and runs like a top.