Oil pressure gauge reading low.

Oil pressure varies with engine speed and with oil viscosity and temperature. As an engine wears, the normal oil pressure will drop. Minimum oil pressure for most engine is incredibly low (almost scary) but for most engines something like 4 to 10 psi or better at idle is enough. With 20-40 psi on a fully warmed up engine, you have plenty. Don't confuse however, oil pressure with level, you can still have good pressure and have low level. Be sure to check that regularly.