I vote for the rocker shafts being installed upside down on an excessive oiling FE!
Have seen this a few times! When under pressure from valve springs etc, the rocker arm is pushed up against the shaft, opening the top of the shaft. If your oiling hole is on top, this is just a gap to pour
oil out of...and it usually gets sucked up via the PCV valve, not to mention floods the valve covers.....
When the shaft is properly oriented, the oiling hole and corresponding slot is covered by the rocker arm radius, preventing excessive oiling!
I'd pop the rocker shafts , or at least loosen them slide a rocker over (if you can) and verify that there is no hole on the top of the shaft!
Cheers!
Steve