Odd oil pressure change

[Yauchout]

I have a CM2350 with 590k original stock oil pump not shimmed 12k mile oil changes. My oil pressure is good at idle right around 28 psi and generally is around 38-39 PSI at highway speeds. recently I see 38 to 39 between 1250 and around 1350 RPM but 1400 and above it drops to 35-36. I realize 35 to 36 is considered safe but I'm not sure why the pressure would drop with increased rpm should I replace the oil pump should I even have any concern. thoughts