Loosing oil pressure after car is warm, thicker oil is not helping

Your problem unfortunately is bearing related. When any oil is cold (regardless of rating) it is heavier than hot. Therefore, cold, with marginal engine crankshaft bearings, you have less bleed off around the bearing faces. Once warmed up, the oil is thinner and prone to more bleed off, lowering pressure. Once pressure is low enough you will begin to hear lifter noise, later, bearing knock. If in the early stages of failure, replacing engine bearings is an option provided that only rod bearings are worn (can be generally done in car. If mains are involved, engine needs to be removed. Oil pump should be replaced during this repair...changing oil pump without repairing the bearings will not fix the problem.