Your thread is a little old now. Just thought I would express an opinion. Since you are able to discern the base oil burning is heavier at sometimes than others it seems that piston, piston rings, valve guides etc. are not involved to any signifigant degree. I would first try to establish if blowby pressure is within normal limits. If blowby is really excessive though you have found your problem. On the newer engines if blowby starts to get into the dangerous area it shuts the pump down. Some people have thought this feature was accidental perhaps but I have thought it intentional design. If not examine any point base oil could be getting into the cylinders very carefully. Never owned one of these earlier blocks myself but mention is made of a diaphram on the injection pump. Suspect the guys mean if it is cracked base oil will be going to where it should not. Basically if blowby is not excessive again I cannot see the basic block being the causitive agent if problem is not constant. I tend to believe you are getting surges of base oil as at a burn rate of 200 miles per quart the engine would not be thought to be fuelling itself on a constant basis with the base oil. Just a few of my thoughts probably worthless.