No valve adjustment?

Cudamark your thinking V8 . The slant 6 had what was referanced in my mopar engine manual as reverse oiling because there was no provision for the needed passages for the oil to travel like the V8's did so they fed the lifters through the pushrods . Ridge 911 it sounds as though the oil of today has claimed ( ruined ) your lifter and possibly the camshaft lobe .....Just a guess though keep us up on the /6 saga !