1969 Chevy Camaro Oil Pressure concern

Hello

I have a custome 69 Camaro that I built myself. Since the build in 2001 I have put over 16,000 trouble free miles on it. As you can see, I don't drive it a lot and it stays in the shop most of the time. The motor is a 400 small block with a 350 crank making it a 377 cubic inch (de-stroke) I have always ran 20-50 Castrol in it with 40 pounds of oil pressure reading on a reliable mechanical gauge. Yesterday I started it for a short drive and noticed the oil pressure was 20 pounds, so I turned around & brought it back home to look it over. The oil level was good & no leaks anywhere so I started it again and it read the normal 40 pounds again. So. I took it out for a drive and the pressure fell to (0) pounds so I killed the motor then satred it a moment later & the pressure came back up but would fluxuate from 0 to 40 pounds but staying around 20. Today I changed the oil & filter but switched grades to a 10-40 weight. Now the oil pressure is reading high around 60 pounds at idle and pegging out my 80 pound gauge at 2,500 RPM's --- I drove it around putting a couple miles on it with no apparent problems but the pressure is very high now. Why would such a dramatic change take place & is this something to be concered about? Could the filter rupture with this high pressure? Thanks, Joe