2001 Dodge Ram no oil pressure

I have a 2001 dodge ram 1500 5, 2 motor with 127000k, I purchased it at a car auction as is and it's definitly a lemon, I already had the distributor and crank sensor replaced. It seem that after I had the distributor replaced. My oil pressure gauge would drop to zero once the motor got warm, at 1st I thought it was the sensor.I replaced it and still the same, I also put 20 50 oil and an extra quart incase the oil pump tube may have fell off. Still the same. The strange part about this is that I can still drive the truck with the oil presure gauge showing 0 as long as I go easy, but one time I accelerated more than your normal driving and got loud lifter noise, I stoped pulled over turned off motor waited a few minutes. Started the motor again and oil presure was fine at idle it only drops when in gear and driving. Although at some point the gauge goes to 0 and just stays there, but if I shout the motor off and turn it back on somtimes I do't even have to wait. The presure is back up. Does change in distributor have somthing to do with this or just coinsedence. Do you think putting straight 40 or 50w oil may help in diagnosing, because it seems the issue starts when motor gets to tempeture and oil thins. Why does the pressure come back up when motor is turned off and turned on. Do you feel it may be a gunk issue althoughoil is clear. Also I noticed the pan is new. Which tells me some one has been in the pan.