my friend has a 91 chevy truck with a 305. it wont start untill the oil pressure is all the way up... so he has to have the key ON untill the pressure boots up then it turns over. he has changed the oil sending unit and a lot of other things like all the feul pumps and filters... what could this be?

 

the problem is likely the fuel pump relay. the oil pressure switch is the back up in case the primary relay fails.