Something Cheap and easy to try if you believe the rings might be sticky:
Marvel Mystery oil results sticky rings
Original thread also has + response
http://www.peachparts.com/shopforum/showthread.php?t=258757&highlight=Marvel
If the Oil was actually burning well inside of the Engine I might not smoke as much as you might think; because since the Oil has become Fuel you simply would not be giving it as much Pedal to go the same speed.
I would be curious and pull one or more of the Glow Plugs to see what type of Carbon build up is on them. If it has a shiny build up I would suspect Oil burning.
I believe it is 2 members who said that the little O-ring in the Fuel Supply/Lift Pump was bad and it caused Oil to be sucked into the Fuel Supply/Lift Pump and mix with the Fuel. I believe one of them said his Fuel had a pale Grayish color to it due to the black Oil mixing with the Fuel.
There has been more than one member who has said that replacing the Valve Stem Seals reduced the Oil Consumption.
It is common for an Engine to use more Oil when it is run at high speeds; but not the amounts listed by the OP.