Truck runs good, it warms up and you stop then the oil pressure goes to nothing and the oil light comes on, but if you put it in park, the oil pressure is ok.... Any ideas on what could be causing this?

1st you should hook up a mechanical gauge & verify the low pressure. If the pressure is good with the mechanical gauge then you have an electrical problem. If the pressure is low, I've seen the pick up screens get partially blocked & cause this problem or all the other problems bad pumps, bearing clearances, etc..........

Great advice from Sidom as usual.

If this is an abused truck - like poor oil changing habits or allowing it to run real low in it's history AND sludge a factor you might buy time if oil pressure CAN show a read OK # by upping the viscosity and changing oil way too often for a while will clear the screen without flash flushing which may cause an issue. Plain quality oil is a cleaner for it's initial time - higher viscosity may up oil pressure into a safe zone all by itself. 15 or 20w- 40 or 50 would/should help for the moment,

T
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Tom Greenleaf - MetroWest, Boston

Replace your oil pressure sensor if you are not hearing any noise coming from the engine.