My old D6 has no oil pressure, no oil being pumped up to rockers. I had the pan inspection cover open and verified the screen is clean and the shaft going in to the oil pump turns with the engine. I checked relief valve setting with another 3 cyl engine and they are set the same (turned all the way in and then backed out 1 1/2 turns). Looking at the parts book, it appears the oil pump drive shaft gear has a pin in it, possible that this pin is sheared? Crankshaft bearings are within tolerance.

I think your on the right cause. If the pressure regulator plunger was stuck closed the pressure would be to high and it is not very likely to stick to far open. I suppose you could put pressure to the oil gallery to see if there is something amiss with the distribution.

The gear that is driven from camshaft has only a pin through it. The driven gears within the pump have key and the upper driven gear also has a pin. If I understand your discription, the oil pump shaft was turning when engine was, so question is was it turning at speed of drive gear or slower indicating sheared pin?

good question!!
not sure if it was slipping, I just turned motor over by hand while watching the oil pump drive shaft. Hope to get it apart this week, will post results.