The average gas engine NEEDS 12 psi oil pressure at idle so @ 20psi you're good! (most idiot light senders turn on at 7 psi which means the damage is done) What brand oil filter are you using. I've seen cheap oil filters drop pressure. I used to work at an engine shop and we had several brands listed that would void the warranty if used (Group 7 filters were the worst and a lot of local garages started using them on their $20 oil/filter change service because they got them for like < $1). The lifter noise probably means the hydraulic lash adjusters haven't pumped up (or down) just yet. Let it idle for a while and see if it starts quiteing down. When we rebuilt the 2.6 and 3.0 motors we would pump the adjusters dry by compressing them in a vice a few times (squeezes the oil out of them). @ 20psi idle oil pressure, it may take some time to get them adjusted.