I mentioned earlier that I would update this thread after 3 tanks of "standard" diesel, giving the fuel system time to be fully flushed out of any 2 stroke oil that I had added previously. Well, 3rd tank refill happened yesterday, so here is the latest set of observations.
Low engine speed torque: now seems a bit sluggish up to 1500 rpm compared to with added 2T; still has a bit more grunt than it used to have at 2000, so maybe injectors/EGR valve have been cleaned up with the 2T?
Quietness: maybe some frosty morning rattle under load just after start up has crept back in. Would need to add 2T again to make a real comparison. Otherwise not noticed the engine being any more or less noisy than with 2T.
MPG: hard to give an exact comparison as the last couple of tankfuls have been different driving conditions for me. Overall though, this has not improved back to what it was before I added the original 2T. So, I was doing around 50 mpg before the experiment started, that dropped to 47-48 with the 2T, now without the 2T if is about the same, 47-48. That's probably a bit too close a result given the unscientific testing (I simply tried to drive like I normally would ) to prove very much, especially given the last set of results with no 2T.
DPF issues: nothing noted here. I covered around 1750 miles with the 2T, about the same without. No warning lights or engine management issues with, or without, the 2T. As mentioned before, hard to tell when the DPF regen has kicked in on the Accord so can't say if the intervals between regens was increased with the 2T, or not.
I am going to use another litre of 2T (circa 3 tankfuls ) once this last uncontaminated tank has been used up, just to do a double check.
My conclusion so far: from an unscientific and non statistical basis, I think most improvements are in people's heads. That is, placebo effect. I am not saying there aren't benefits, but none that I can really perceive if I am honest. Surely, if people think adding 2T will damage their engine, they wont do it, so you don't hear from them. If people think there could be a benefit from adding 2T, then they end up victim of the placebo effect?
I am happy to invest a further £9 for 3 more tankfuls as there seems to be no downside to this, so I can conclude for myself whether I will continue to add 2T or not.