low oil pressure???

my truck, 91 s 10, has had a problem with the oil pressure gauge...at first it hovers around 40 and after it warms up the pressure gauge drops all the way down causing the "check gauges" light to come on...it's been doing it for a while now so i don't know that the pressure is really low or if the sensor has gone bad..any suggestions?

What weight oil are you runnin?

usually bad oil sending unit