I think you need to thoroughly clean your EGR valve. I believe what is happening here is the valve is stuck open with oil/soot deposits. This is allowing lots of exhaust loaded with oxides of sulphur and nitrogen into the combustion chamber and generating very high levels of nitration/oxidation. Both these processes result in the formation of organic acids, which is depleting the TBN. In addition, oxidation essentially increases the molecular weight of the oil and is causing the thickening you see.
The wear rates look pretty good, but running with very high rates of oxidation will result in the gradual formation of sludge/varnish/carbon deposits. So this isn't something you want to let continue. It is very uncommon to see Delvac 1 degrade like this in the TDI ...!