Straight weight 50 vs 20w50 will do nothing. Its 20w vs 50w cold, not hot. What you have is enough flow paths hot to reduce pressure when the oil is thinner due to temp. So is the oil temp too high? Or is there to much internal leakage? First off I'd be measuring oil temp. Water temp seldom correlates to oil temp. Is the oil temp in range? Then you have a decision. Either your build has excessive leakage in the lifter bores, valve train, drain back issue, etc. Or the engine is in trouble. If you're confident there is no issue, the high volume pump will help. Make sure there is sufficient capacity in the pan.
Doug