You should have oil right away. 30W should be OK, I don't see why not. This is a hard call not knowing anything about your engine build, but you can check a few things. x2 with BearGFR if the oil plug has been left out at the rear of the block. It's hidden within the freeze plug at the back of the block.
Possibly the wrong lifters. Many times you are given Chevy lifters which are not correct
even though some of the parts books will say they work. The oil groove is not correct and the oil may not be flowing as it should. Check this comparison out: http://www.jbp-pontiac.com/techDoc/PontLifters.pdf
Is it one side not getting oil or both? There are 2 oil galley plugs in the front of the block near the timing gear. If inserted to deep, they can cut off oil supply. Typically when these are replaced, they get tapped, threaded, and screw-in plugs inserted - but not too deep.
If the rocker arms are too tight this can cut oil supply off to the rockers. The lifters spin on the cam lobe and you should typically see the pushrod spinning as well when the engine is running. Although this is for poly locks and not the stock Pontiac rocker arm nuts which get torqued down, you can adjust them as instructed by spinning them with your fingers. This will get you where you should be without torquing the rocker arm nuts down. Then, see how much more is needed to torque the nuts. If you are right on the money, the pushrods should still spin as you torque the nut, then you are good. If you have to tighten them up beyond that, then it may be too much and the pushrod will not spin and you could be cutting off the oil supply in the lifter. Read this: Pontiac Rocker Arm Adjustment
Seeing you have 65 lbs of oil pressure, it may be the wrong lifters. I would not think you would have good oil pressure if the oil galley plug were left out near the distributor - but never had that experience so cannot say for sure.
In any case, you want to address this sooner than later or risk damage to your engine.