Oil pressure comes from the oil pump and the bearing clearances. It's a fool's errand to think those bearings aren't worn. The engine is out and apart. Slant 6 or Briggs and Stratton, it needs new camshaft bearings. If you were local to me here in Georgia, you could bring it by and I'd knock new ones in to make a new friend. It's a 30 minute job taking my time.