It is normal for the oil pressure to take a few seconds to build to full pressure. After sitting, oil drains down from all the oil passages, and may also drain the oil filter, so when you start after sitting, it has to re-pressurize the entire oil system. Nothing to worry about, unless diesel has diluted the oil viscosity.
Moved to LB7--
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.