Oil pressure changes with engine rpm too. Assuming (because you didn't state so) that it drops to 15 psi at idle, that's not too uncommon or alarming for a warm engine. I personally like 20 better, but 15 isn't bad. 10w-40 is not real uncommon, though I prefer 10w-30. I could go into a long discussion on why, and how, but I don't have time for that, and if you did a search you'd find earlier discussions on it anyway. If the lifter noise, assuming you've diagnosed that correctly, is oil caused, it could be that 20w-50 would help too. But that is a guess. It could be that there is something blocking the oil galley that supplies the lifters too. Or you don't have the rockers adjusted correctly.