It is my understanding that those oil restrictors above the mains are part of the "priority main" oiling system. So if you loose pressure to the cam or lifters; the mains still have some oil pressure to make it live until engine is shutoff. It can change the running oil pressure measurement in the lifter bores & cam bearings compared to the mains. I was pushrod oiling my KB motor, and the pressure in the lifter galley was just right so I didn't need to restrict oil flow to the heads.