Just a quick update on the Z18XER oil consumption.
I was monitoring the oil consumption for a couple of weeks during my urban commute to work and the oil level didn't budge. Part way through a journey back to Stoke the oil level warning light came on which was quite alarming as this was considerably higher oil consumption than before the piston soak. Thinking that I had probably made things worse, I topped the oil back up and thought that I was going to have to strip the engine down. However since then i have done a number of motorway journeys and the oil level hasn't moved.
I was thinking that the piston soak partially freed the rings but also removed some of the sludge/coke buildup causing an additional oil loss. The high revs during the motorway journey helped to free the rings which are now controlling the amount of oil correctly.