If I had to take a guess, I'd say someone did not change the cam bearings OR installed them turned wrong (it's easy to do, been there myself), and I also had a guy bring an engine by that was a fresh rebuild with less than 1,000 miles NO oil pressure. The builder (who was supposed to know what he was doing,) did not use any assembly lube. Needless to say the bearings were all junk. But I am completely guessing. I have had good luck with "Hi-tach" and Lucas products for quieting down noises and increasing pressures a bit. I run one or the other in all my high mileage engines, but just my input on it. Either way you most likely have an internal problem and will eventually have to pull the engine, however the thicker oil or additive will buy you sometime. Again ALL of this is JMHO, Best of luck with it
Kelly