I know I just missed a thread on this subject, but at the time it wasn't
info I needed. Is a FAQ available on this? In any case here is the problem:

If I let my 78 Van sit for more than a week or so, it inevitably develops
a loud tappet noise on one or two cylinders. The last time it did this
I put some oil treatment in that was supposed to free loud or sticky
hydraulic tappets. I then changed the oil to 20-50 Castrol, and the
problem went away after a few minutes of running. I haven't been
running the beast for the last few weeks, and now it is doing it
again on two cylinders. Any suggestions, help etc.?
How'd you fix this?