Tsiero said "This engine used 5 qts of oil between NC and New Mexico about 1700 miles" yikes! something's wrong! I use one qt per 1200 miles in my '87 300D turbo and I consider that a lot (246K miles on the original rings and a #14 head BTW
) My '85 300TD uses one qt per 1500 to 2000 mi.
Hopefully just the turbo bearings are leaking, if there isn't a lot of oil loss visible anywhere I would look there. And a new cartridge for that turbo might be inorder. I have also seen 603 engines lose power because the wastegate flapper sticks open due to crud built up and caked up in the space where it should close. A turbo shop is another way to get one reconditioned, I have no interest in repairing a tuebo (basically a chicken I suppose, but worry about the need for balancing) but my friend tackled one and rebuilt it successfully.
Watch out for loss of oil so as to avoid running the sump dry
and let us know what you find was the cause