I've used .120,.080, .060 in the stock oil galleys to the heads using ductile iron rockers and .039 in my Indy headed motors with Harland Sharp and T&D roller rockers and a grooved # 4 cam journal I help a local racer with a set of Indy 440-1 that fought the same problems on his motor on the dyno, loosing oil pressure do to insuffeceint oil return so I started my Indy headed motors at .039 in the pressure lines to the heads What type rockers are you using now? The World iron blocks have a .040 restritor in there oil galleys to the heads