Not a dog with a bone syndrome. I am wide open to others thoughts. My thought is rather than differant amounts of fuel fill and that is probably occuring. The principal suspicion is the sequential timing is changed as any element with getting less fuel.The injector will open later than by design.

The lift pump is a constant pressure type but at the same time only recharges once per injection pump revolution. Filling the later elements in a a really low fuel pressure initial setup may mean they are almost scavaging from the decreasing low pressure profile.

Since the relief valve is no longer functional. There is nothing to provide a constant fuel pressure through the remaining fills until the lift pump recycles again.fuel most likely is not able to flow through the filters quick enough to make up the deficiency even if the constant pressure is still available at the lift pump.

As I have said I feel this is a long term deficiency problem that wears the first cylinders rod bearing faster. Why I know something is going on is the engine sounds different with different fuel pressures to the base of the injection pump. The power balance of the engine sounds better with good pressure as well. . Anything you can actually hear has got to be signifigant.

Plus in corrected situations there seems a little more power available in the seat of your pants. With that relief valve closed because of not enough pressure to keep it open. The pulses that are generated inside the base of the injection pump are fierce. How and if they contribute to the issue is another unknown to me.

Now back to your oil viscosity issue.I am no rocket scientist. Say you have a car on 10-30. The car is burning a quart of base oil every 300 miles. You change to a straight 30 weight. Your consumption on average will drop to 5-600 per quart usually..

The only change that seems reasonable to me is the viscosity in effect must remain better. The multi grades state they protect to the same. Still it almost cannot be on a true overall viscosity basis.

The 10-30 has a polymer type component that changes with temperature. The molecule size of it might be much larger in any temperature state than oil. Or only at higher temperatures.

We all realise that a 240d on 15-40 will usually consume more oil than normal on the highway. I attribute this to the higher rpms on average it must run at in comparison to the 617 for a usable speed. I do wonder if the same engine was on straight 40 if the consumption pattern would be less or more on the highway. I suspect it might be less. Just not certain of that either.

Another thought along this line.If you are burning multi grade oil. Is the polymer being burnt up at the same rate? If not by continually adding oil you would have an increase overall of the polymer component. That may be good or bad. I just do not know.

The producers of the multigrade oils have not been particularily open about the whole picture. My wifes car for example takes a 0-20 synthetic oil. To keep the warrranty valid they want only certain oils used. This has led my to wondering why the synthetic oil must be special. Plus what is in it that is different than the normal run of synthetics.

The best yardstick I have been using to determine the probable life left in my engines is the oil consumption rate for a long time now.There is a serious problem with wear. Once it reaches a point it almost exponentially increases.