Land Rover Repair: excessive oil usage - Disco II, crankcase ventilation, head gasket replacement

Question
I recently had my independent shop replace the head gaskets on my 2004 Disco II due to an external leak on the driver's side.  Prior to this leak, my engine burned two quarts of oil in between the 5,000 mile service.  After the repair, my engine has burned through 8 quarts in 1300 miles.  I took it back to the shop where they performed a compression check to make sure it wasn't the oil rings (all within specs - like 120 range), then after finding that the rings were good they pulled the valve cover gaskets and checked the valve seals - all were in place.  Their only note was that the No. 8 spark plug was ashy. I am running Amsoil 10W40 full synthetic, and do not see any smoke nor is the oil leaking onto my driveway.  The shop even took it for a test drive with someone driving behind and saw no smoke.  I do, however have black soot on the tailpipe.  We're all at a loss as to why the engine is loosing oil.  One theory is a bad/incorrectly installed valley pan gasket - thus allowing a bit of oil from the valley into the No. 8 cylinder.  Any comments?  HELP!!!

Answer
There is no obvious reason why oil consumption should increase following head gasket replacement.  If the oil were getting sucked in from the valley as you speculate I think you would have a check engine code for air leakage.  You say the valve seals are in place . . .  the next place I would look is the crankcase ventilation system. That would be my next guess.   Black soot on the tailpipe is normal and not suggestive of anything.  

If the oil consumption truly skyrocketed after the repair I'd look at crankcase vent for  sure.  One more point . . . you ruled out each of the major causes, but with all due respect, the most likely cause of oil consumption AFTER a head gasket job is some kind of workmanship error.  So I'd go back and look really carefully at what you think was done.

We used to see piston ring failure that produced oil consumption like that but it would only follow overheating.

Good luck