Losing Oil Pressure

I just bought a 97 S-10. When I start it up I have 40lbs of oil pressure. As the vehicle warmed up the pressure would eventually drop to zero and the check gauges light comes on. Going down the road at 50mph, no oil pressure, push in clutch and rev engine and oil pressure rises but will not remain there. I changed the oil because the oil in there seemed thin. I now get lots of pressure and it will now drop to between 10-20 lbs. If I am at an idle, 1000RPM, the pressure still drops off to nothing. Is this a common problem? Truck runs real good. I was also getting puffs of white smoke several times out of the exhaust. No sludge or water pudding in the oil. The oil was very clean when I had it changed. I know that my bros ranger would not run on synthetic and also had no oil pressure with Syntec. Any body have any ideas. I also stopped getting the white smoke after I removed the radiator cap and put it back on. Cap allowing more than 15 lbs causing leaking into piston chamber??? Are the oil pressure and white smoke related?? Can I use Bars Leak in radiator to help seal head gasket?? Do I need to do the oil pump or is it the gauge??? 130K on truck.

Are the oil pressure and white smoke related??

Most likely. It has engine damage, by the sounds of it maybe a coolant leak/overheat caused engine damage. Get a coolant system leak down test, check your compression and see what you get. It looks like though, its time for a rebuild.

With the oil pressure at speed and none at idle, sounds like engine damage. No oil pressure equals seized engine.