Ventiation problem wouldn't cause the oil pressure to skyrocket, however if the cam bearings were in stalled incorrectly that could cause the high oil pressure. Tight bearing clearances could also be cause to high oil pressure, but would eventually fail. I would first try 5W30 oil. It's also possibly that the distributor housing is blocking the flow to the top end. Have you looked to see if you're getting oil to the rockers :?:
Not advice just wondering.
Zip.