When I had my 1998 Tahoe with the Vortec 350, my cold oil pressure varied around 10 psi depending on oil brand. Quaker State UD and RGT would be 40+ psi and PP would be just over 30psi. Ran the same Pure One filters and same 5w-30 viscosity the whole time. Had same results when switching back and forth between the brands. Never had any drivability or changes in engine noise, so never really worried about it. Sold the truck with 260k miles on it.