let me tell you where im at, I just got a 73 chevelle that has been sitting in a barn for about 15 years, the engine was rebuilt before it was parked and has about 10,000 miles on it. The oil was like hot fudge when I got it. I changed it , and oiled the rocker arms and down the pushrods before starting it. It seems like some of the rocker arms are not getting a whole lot of oil compared to the other ones. Oil pressure is 40 when cold and can drop to about 10 in 100 degree heat. The engine runs good, no smoke at all and after about 2000 miles of driving the plugs look like I just took them out of the box. The only problem is the clacking coming from the engine, I have not done the preload on the rocker arms so that may be some of the noise, but im concerned about the lack of oil to some of the rocker arms, I think some of the lifters are dirty and not letting enough oil through. Thats why I wanted to swap them out since I was going to change the manifold but I dont want to ruin the cam. Now you guys know where Im at what would you guys do?