You either have low oil pressure, or a defective oil pressure switch for the light. These engines did have some problems with early oil pump failures, so it would be a good idea not to run the engine untill it has been diagnosed and repaired.
Unless you ahve an oil pressure gauge set, I would suggest taking the truck in to your favorite local repair shop right away to have an oil pressure measurement done right at the engine, to find out if it is an oil problem or a switch problem. If the engine is indeed low on oil pressure, allowing it to run will destroy it very quickly, requiring engine replacement. If it is a light problem, repair usually costs well under $100.
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If the oil light is coming on, :then either:
(1)the engine oil pressure actually is low (faulty oil pump, clogged pickup screen, internal engine bearing wear, or diluted engine oil)
(2) THe gauge is reading inaccurately due to an out of calibration sending unit, poor electrical connection, or faulty gauge.
There really are no other possibilities...
are you saying you had the oil pressure measured with a mechanical gauge when the engine was hot and at idle and it was normal? What was the pressure reading?
Engine oil pressure normally drops as the engine warms up, due to the oil thinning out as it gets hot; you see this even on a new engine. That is why oil pressure measurement specs are always given at normal operating temperature (195 degrees F)
What is unusual on your engine is that the oil pressure approaches zero when hot; idiot lights do not typically come on untill around 2 psi, at which time internal engine damage is occurring. Typically, this is due to either a failing oil pump or internal engine bearing wear. These engines seem to have a high oil pump failure rate, causing oil volume to fall at idle when the oil is thinned out on a hot engine.
The way to find out what is happening in your engine is to have a mechanical oil pressure gauge connected to the engine, run it at idele hot, and see what the actual oil pressure measurement is. If it is truly near zero, then the next thing to do is to install a new oil pump and screen to see if the pressure returns to normal. if it does not, then the engine is worn internally (incorrect bearing clearances) and will need to be disassembled for repair.