During a pre-trip inspection with the engine idling, what oil pressure gauge reading indicates the engine is operating within a normal range?

AAMVA CDL Manual (nhcdm.pdf) Figure 2.5, Section 2.1; inspection required under 49 CFR 396.13

Oil pressure gauges are one of the first things drivers check after starting the engine because a failing oil pump or oil leak can destroy an engine in minutes and strand a loaded rig. Knowing the correct idle range versus operating range keeps drivers from either missing a real problem or shutting down a truck unnecessarily.