Reply to bob:
Not completely worn out Paul but the shaft seals are prone to leak oil as they get more hours on them. With the consumption dropping down near normal now the seal rings might have cleaned themselves with the oil leaking by them. 2 L a day isn't bad, if it goes back up check pipe from turbo to manifold also take turbo oil drain line off and wash it out. Saw a turbo once installed with the plastic plug left in the oil return hole, turned the hood into a black oily mess on start up.
Later Bob
Paul - 2 litres a day is pretty acceptable oil consumption for an old D8. You might find changing your oil brand, and in particular, using an older spec API classification will reduce oil consumption. Also, ensure your tractor is working at correct operating temperature, and is not being idled excessively.
The newer API spec oils are supposed to be backwards-compatible with the older spec oils .. but the new spec oils have very high levels of aggressive detergents that are hard on the old tractors .. and the new oils make their way past the old tractors piston rings a lot easier.
Many years ago, when I was full-time contracting, we found that Mobil oils performed terribly in Cats. You could have a tractor running on another brand, and using little oil .. and if you swapped over to Mobil .. your oil consumption would rocket to 5-6 litres a day, from say, 1 litre a day.
It was something to do with the "slippery" additives that Mobil were using at that time .. but you could always tell a Cat running on Mobil oil, from the tell-tale blue smoke at idle.
I wouldn't use the Bon-Ami treatment except as a last resort. It does work .. but you need to be sure that the liners are glazed as a result of excessive idling, cool running, or the wrong oil type.