Where is the oil coming from??

In the early part of my engines life,while using 10/15W40, and when redlining it, it liked to blow oil out from the oil filter area. It was not ballooning the can. I figured it was blowing the shaped filter seat off the block. (no 90* adapter). So i grabbed a second plate, and stacked them. I doubled the number of holes in them by drilling and chamfering.This cured it. Some time later, I switched to 10W30. Been that way now for about 10 or 12 or more years.