I just bought a 1990 f150 it has a 5.0 engine, truck ran ok when i bought it 155,ooo miles now after i derive it about 20 miles the oil pressure goes down to nothing and the engine light come on, it has a check enging and engine light...my question is this the oil pump or something more serious...also if i rev the engine to about 1100 rpm the oil pressur goes back to mormal...

thanx guys in advance for any help...

Check it with a mechanical oil pressure gauge, might just be a bad sending unit.

I agree, check it with a mechanical gauge. But at 155K I wouldn't hope for much. If it truly is low at idle your main bearings are probably worn. Try heaver weight oil it will prolong life a little