oil pressure

I drive a 2011 serria 1500 with a V8 6L. I was pulling an rv up a hill and the oil pressure dropped to zero I pulled over and checked the oil using the dipstick and it showed full. The oil was dark and thin I use 10w40 synthic. The water temp was about 300 to 320 degrees. What could have...

I have a 1998 GMC K1500 Z71 offroad with the Vortec 5.7L 350ci V8. Cold air intake True dual exhaust (no cats, dual in/single out Magnaflow muffler) Nothing else done to the engine this far. Idle is 600RPM, my oil pressure fluxuates from nearly 0 at idle and back to normal at WOT. Not sure...