If the oil pump was inspected, end clearance and to the housing clearances are good and no big gouges, it is not likely the issue. Now the lifter style may be. If they pressure feed the rollers needles, that introduces another "leak". If they are splash lubed, that is not the problem. If you have the pressure fed roller needle style, a high volume standard pressure pump will help bring the idle pressure up.
This is like science, you have to analyse all the factors and consider changes made. I hate throwing parts at a problem that are not fixing the problem.
