You say you do not use any oil at 3000 change? how about water use?
That engine sounds pretty tight,if you are not showing oil lose at all then I would tend to think that the valve guides are still in good shape.My 420 used a half quart in 3000 so I am looking at guides and seals for the same. Water use can tell another story,if you have to add is it a leak up front or no noticeable leak,one is obvious and the other maybe internal lose and thus steam at the exhaust,If the later then you are going to have to do heads,it could be as simple as a head gasket. The next thing I would look at is The exhaust system clogged cat will cause a slowing of the speed and poor fuel mileage,unhook the system and see if it improves at idle, the next is the EHA,check to see if it is reading in spec,lots of post on it,The HEAT must go do not let it build change the thermostat and flush the core to help the flow, let us know how you are doin JNT