First check that the lift pump is functioning when the engine is running, if not the oil pressure sender that also makes the pump run could have failed. If the pump runs, then there is air getting into the fuel line before the lift pump.
1995 6.5 turbo GMC K2500 extra cab 4x4 pickup, PMD under bumper, mechanical wastegate, 228,000 miles, rebuilt engine, 4.10 diff, 16 mpg
1994 6.5 turbo GMC K2500 4x4 suburban "F" VIN #9 resistor, PMD in the bumper, mechanical waste gate, 226,000 miles, Positrac 3.73 diff, 17 mpg highway.
1982 4x4 shortbed 6.2 Chev 260,000 miles untouched motor 17 MPG 4.27 diff. Best thing I ever owned, 21 years.