Truck Repair: low oil pressure in N14 after 180 oil temp, oil pressure sending unit, low oil pressure

Question
2001 international 460 n14 650000 miles
3 days ago had regular service lube left next morning drove 20 miles came to a stop, low oil warning, oil level ok, replace filter again, replaced oil again, replaced oil pump relief valve and spring, replaced oil pressure sending unit, shop has tested pump pressure/flow within specs, last oil filter shows no metallic debris,

engine oil temp        idle            1500 rpm
cold                  32 lbs             40 lbs
1/2 warm              25 lbs             30 lbs
180 +                  3 lbs             25 lbs

tomarrow they will disassemble oil pump to inspect, then maybe drop pan to inspect stuff, and they have yet to pull all valve covers and inspect. problem started over night with no warning. also pressure guage needle flucuates 3 to 4 lbs at all pressures and rpms (never did this before. normally at 180 + oil temp 1500 rpm under extreme load produced 32lbs oil pressure, more on newer oil.

do you have any suggestions on how to diagnose or what to look for?

Answer
Same brand & viscosity oil?That would be my first check.Take the temp sensor out of oil pump & install a gauge there.Should have 65-75 PSI at high idle.Then check before filter & after filter,then check at main oil rifle on drivers side.Pressure that low will be shutting you down & logging a 143 fault.Before removing oil pump to disassemble,remove suction tube,get a large screwdriver & wedge the oil pump gears.Get someone to MANUALLY turn engine with ratchet & see if oil pump gears are slipping.