Oil pressure drop at high speed

The pump moves so much oil at high rpm that most of it ends up in the rocker covers and valley...the oil can't return to the pan fast enough to replace what the pump has pushed upstairs which leaves very little oil in the pan .
your rocker arm oil return holes could be stopped up some what with sludge and not letting the oil retuen fast enough to the pan same with the intake valley
take a rocker arm cover off and see if this is the problem , if its all sludged up in there clean it out on both sides making sure the return holes ae fully open and clear of sludge and try it out , if it hepls some you might want to pull the intake off and clean in the valley as well