I have a Yanmar YSE 12 inboard diesel engine approx 50years old?

The oil presure light is intermitentley on and the electrical oil pressure gauge is not showing any pressure. The oil in the sump is at the correct level .
I am getting a mechanical gauge for next week end to temp screw in to oil pressure switch tapping (what is this thread). What is the normal running oil presssure for this engine. I can sort out instrumentation problems. Are there any Known problems for no oil pressure on this engine?