1999 Chevy S10 oil pressure drop.

Is your vehicle running properly ?Have you removed the valve cover(s) to see if oil is making it's way to the valve train ? Maybe a faulty gauge ? If not...........If you primed the oil pump and oil is topped off, it may take a partial dissassembly of engine to determine where an internal oil leak is.It might be worth having a pro look at the engine, esp.if you are planning on keeping the vehicle.I know this is a rare occurrence on the 4.3 engine, so my guess is you have a 2.2 in that vehicle.Either way, suffice to say, sounds like an internal oil leak.If not a faulty gauge.Bad seal, gasket , mating surface, etc.

I did not see 4wd in narrative......it must be the 4.3 litre.It is a good engine.First thing I'd check is the gauge, if vehicle is not showing signs of distress.

yes its 4wd

Since you changed out the sensor, I would guess you made sure the connections are good. Try adding an oil additive for older engines or putting in say 40 or even 50 weight oil to see if that helps. Older motors will wear-out bearings and such so will not hold the pressure as when it was new and not worn. How many miles on the 16 year old truck? How was it driven? Normal to and from work and dates, raced in the mud fields or used as a work truck in dirt and dust year round? Change oil and filters often as recommended? Is the wire to the gauge good? I'd spend a little money to find the issue before spending a ton of money on an rebuilt or new engine.

If it's a high milage motor like mine (201000) and you have oil presure when it's cold odds are its just worn bearings not much you can do short of a rebuild.