Oil pressure warning light:

Oil pressure is also monitored by an oil pressure switch that puts a light on when the oil pressure reaches a low level. The light is usually red and will be incorporated into the aircraft warning systems to alert the pilot. On later aircraft the pressure switch may have two pressure switched within it. A speed comparator will decide which switch to monitor. The idea being that a low oil pressure of say 20 psi is fine at low engine speed, however at higher engine speeds the engine could be sustaining damage due to insufficient oil pressure even though it is above 20 psi. The second pressure element would be activated when the engine speed was greater than say 80% and the oil pressure less than 50 psi.