I've been having the same issue with my Z18XER daily. It uses a considerable amount of oil on motorway journeys.
When I bought the car (from the old dear), I serviced it and took the rocker cover off I noticed oil sludge build-up in the corners of the head (this car had been serviced by Arnold Clark through the service package that she purchased with it).
Since the service, i kept getting the oil low level warning on the dash after a motorway journey. The old dear said that she never had the warning light come on which lead me to believe that when i dropped the oil, the sludge was dislodged and blocked somewhere else.
After numerous engine flushes and oil changes, the oil consumption seems to be slightly better but still not great.
Think i may give the piston soak a go and see if that helps.
