my 1989 chevy pu 350 gauge light comes on & psi gauge drops 5

check oil level? check oil puddles under engine? check oil pressure sendi ng unit?

first stop is oil level. Then oil pump.
Could also be the pressure gauge.

the check gauges light is triggered when the oil pressure drops to less than 5 lbs.

sounds like when the engine gets to operating temp the oil is not thick enough to stay where it needs to be (usually main bearings)

assuming you are full of oil, I would try changing it out, and running a heavier straight weight oil (depending on your climate)

an oil pump might gain you some time, but I doubt it makes it all better.

also double checking the oil pressure with a remote mechanical gauge is not a bad idea. the sender on those is sometimes wack...

good luck.

is your motor diesel or gas ?