Oil Pressure light goes on when you cold start your carbut goes off about 20 seconds later why?

Light is supposed to come on as soon as the ignition key is turned on. It should not take 20 seconds for the oil pressure to reach enough pressure to turn the light off. Change brands of oil filers you are using and make sure you are using the correct weight oil for this engine. Try a Purolator. If that doesn't help, check the oil pressure sending unit. It that checks out OK, have the oil pressure checked to see if there is a problem with the oil pump.