I have a little experience of excess oil consumption myself.
The first was my own 16v 1.8 155. Around Christmas/New Year, I used the car over a couple of weeks very
hard. Cut a long story short, after 500 miles, the sump was empty and engine damage occurred (noisy variator, 1.5bar lower oil pressure and now a voracious oil appetite).
What I always wanted to know was if the oil consumption was cause or effect. I checked the oil level before this 2 week period of hard driving and it was right up. 3 months earlier a trip to the Nurburgring used 1/2 litre of oil, and yes, 120mph was exceeded on the way, and many times on the circuit.
The other car is a 146 Junior. Used sensibly as a commute by a middle aged woman. As she only had it less than 2 months, her experience of oil consumption was very limited. That said, it didn't appear to use much oil until one day, it was halfway down the stick. It was topped up with half a litre of 5W/30- yes I know it's wrong, but the next week, it needed 1.5 litres of 10w/40. The day after, it started knocking one the way home. The result was high oil consumption (very sooty exhaust pipe) and damaged big end bearings. The oil level was not run below the minimum.
The wrong oil may have been a factor, but my thinking is that it would dilute the oil less than an engine flush would.
In both cases, the cars were less than 4000 miles after an oil change (10w/40 used in both cases), and I think both cars had negligible oil consumption previously. The 146 had an EGR valve- the 155 did not. Both cars suffered very sooty combustion chambers, spark plugs and tail pipes.
The 155 was used hard but the 146 was not. The 155 had 120K whereas the 146 had 55K.
I still have both engines (in cars). As economical repair seemed not possible, I have not stripped either engine, but with suggestions, I would like to get to the bottom of this, because, after asking around, it seems that a number of thes Twinspark engines appear to suffer some sort of failure which causes sudden very high oil consumption.
As a last thought, changing spark plugs would only allieviate things temporarily and not cure the oil consumption. Both engines still have very good and even compression pressures, and do not produce much crankcase gas- even under load. The air filters are clean, and the oil seperators for the breathers do not appear clogged.
I've run out of ideas....
Anyone who wishes to speak to me at NAD can do so. I'll be with the Scottish contingency and I'd be grateful for input. I'll also pop by the 155'er or AO'er if there's a special area.