ok distilling the facts from the fiction...

car 940 turbo, b230ft, garret turbo, 135k

the turbo uses 1 ltr oil per 100 miles or so if driven hard..

It has had the oil leaks sorted, a new turbo etc..

The engine compression tests as follows :-

dry 155,150,135,140
wet na,170,165,190 na=couldnt work out how to get oil in there!

breathers etc are all (now) clear..they were all clear, but the oil return pipe to the engine was VERY coked/sludged up (possibly blocked!)..now clear and i can (slowly) pour oil down there...and get a peice of winscreen washer type tube down it!

The cat has been swallowing oil for a while (eg. with old turbo - although changing the turbo didnt alter the oil use!)

The exhaust has a 'crush' in it imediately before the mid box restricting the pipe (possibly to 1/2 - 3/4 or original volume)

car runs well, but if you follow it under heavy acceleration (4500 rpm full boost) it does smoke a little bit...not clouds but a small amount of darkish greyish (maybe blackish)smoke - certainly you have to be looking at the exhaust to see it!

At idle the car doesnt smoke (At all - even after extended periods)

A bucket of water with the exhaust palying over the surface at idle indicated no oil on the water surface (or if the engine is reved and held at circa 2500 rpm)

The oil use is MUCH worse on long high speed (70-80 mph) turbo using journeys, round town it uses much less (maybee 1/8th - 1/4 ltr per 100 miles)

there is oil in the turbo tubes...seems to be being drawn up the breathere tube from the oil seperator.

The air filter is newish (ok) and no restictions in the intake pipes...

17" vacume at manafold at idle, idle at 2500 rpm a couple more inches of vacume & rock steady - no decrease just sits there...(indicating no serious exhaust blockage?????)

spark plugs are nice and tan/grey, but cokey round the rims

exhaust is sooty

if you disconnect the manafold to flame trap and measure the presure on the flame trap nipple at idle you get about ambiant presure, rev the engine and it dips some...not a lot, but a definate dip...

Now given mobil 1 doesnt evaporate and im not leaking it (honestly im not!) its being burnt (and at a rate!) so principle possibles are:-

1 valve seals (how to test?)

2 rings/pistons (rulled out by compression test - for this much oil???)

3 turbo seals possibly due to excess back presure from cat/crushed exhaust?
cat looks original and NO cahnce of removing or geting the plug out without
an OXY!) the nuts are pyramid shaped!

4 Blocked breather oil return pipe leading to excess oil in breather (now
possibly cured - (could this have the effect i am seing????)

What i am looking for is (a) a consensus of opinion of which of the above is possible/likely (and why), which is most likely and what OBJECTIVE testing I can do to rule them in out...

I have no objection to spending sensible money on measurements/tools if they will help me *diagnose* the issue (eg exhaust co2 analyser)

also how much do turbo's just smoke under heavy boost (given a posible oil covered cat/oil in pipes etc)

Finally....HELP! - im running a volvo on sythetic 2 stroke in effect and it isnt cheap!