I wouldnt be surprised if lifters are collapsing and getting hammered. 5W20 seems thin for a hydraulic lifter on a BB with hyd roller spring loads.

Performance lifters are often made with more plunger to wall clearance as a fix to prevent pump-up at high rpm when the effective plunger load on the closing ramps become very small and valving response time is at or above its limits. (Of course this is happening as oil pressure is at its peak).