My 78 pickup (20R engine) has the oil light flickering dimly, while the oil level is fine. I tried adding some oil but it made no difference. It goes off completely while the engine is at high revs, but flickers while the engine is at low revs and in gear. The light goes out at idle. Is this a sign of a bad oil pump? Any other potential guesses? The engine does leak oil from the timing cover and main seal...

 

That old of an engine...probably worn bearings on the crank...the higher revs spin the pump faster which increases pressure. You can run 20/50 oil in it and it will last a while longer...