Got a quick 300SDL oil pressure question.

[LUVMBDiesels]

In my 'new' '86 SDL, the oil pressure never gets above 3bar. It starts cold at 3, idles warm at around 2+ then when I press the accelerator comes up to around 3, maybe just a hair lower than the number. This happens under load at all RPMs off idle.

I know from experience with my 300SD that the 617 should bring the oil pressure to 3+ at any RPM over 1800. Is it the same on the 603? I just changed the oil from whatever the PO had in it to Rotella synthetic 5w40 but the pressure is the same.

Could it be the sender or gauge reading low? what voltage should the sender be putting out at 2bar, 3bar and over?

Thanks!