Hi fjtwelve
I have been reading this thread on and off for the last 12/18 months and have thoroughly enjoyed it, I really admire your determination to get the problems sorted, and they seem to come one after another these days, probably the high mileage may have a factor on some of them.
Like you I run a V6 2.5 tdi but mine is in the form of a late 04 Passat Estate, last of the old shape model and has only done just over the 100,000 mark so hardly run in yet.
I also have a couple of add on's to improve things a little in the form of a german DTE tuning box which gives around 30bhp increase a water/methonal injection system and the customary K&N filter change and thats about it. It goes extremely well very torquey and will easily return 45+mpg on a run with a regular 40----ish locally.
The only downside to it all is the amount of smoke it puts out, I give a good blast from time to time to clear it out especially if her-indoors has been "poodling" around locally for a week or so, and when I do it fills the road behind so much so you cant see for a couple of hundred meters or so until it clears, but it is always there when the boost comes in.
Anyhow to the point, I have recently been reading a lot about the adding of two-stroke oil as an additive to your diesel tank, with I have to say very favourable results, so I decided to try it last weekend I didn't know the recommended ratio then which is 1:200 so guessed it at about a cupful to around 45/50 liters which was rougly what was in the tank at the time, turns out I wasn't too far out.
After running it now for a few days and adding around 150miles to the car it's amzing the difference it's made the car is so much smoother, noticebly quicker and the best to come the smoke as dissapeared just about completely, oh and apparently you get an improvement in mpg, which remains to be seen but I'm optomistic.
By the way I'm just up the road from you by around 10miles give or take a yard or two near Redruth.