Can any body tell me why my 1987 f250 makes the oil pressure go zero n then turn it off?

Hello people I have a problem with my 1987 ford f250 months ago I made the maintenance of oil and filters the next day take it out 30 minutes after low oil pressure went to zero and turn off. Returning home I parked the truck and four hours later when the truck was cooled i turned on well except the pressure was low and only lasted another 20 minutes turned on and off since that day remains the same. Not heated truck someone can tell me what's going on I can do?

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