I also recommend a compression test. I could indicate a head gasket failure, no uncommon with the ol 6.9l. Or possibly stuck rings.
If you have the valve covers off at some point, just double check the head bolt torque. The 6.9's required a "re-torque" at 30,000miles or so after leaving the factory. People who failed to do so are now experiencing mass head gasket failures at 200,000miles or so. Ive seen 6.9's in industrial and commercial use that have never had any head gasket issues and are pushing ridiculous amounts of miles now...the heads were all re-torqued as required.
Just something else to look out for is all 
Good luck Buddy!
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1984 F-250 6.9, T-19, BW1345, D70 rear end. solid truck.
1984 toyota BJ60 Landcruiser, 3B diesel, H55f 5 speed, Best rig Ive ever owned. Never needs anything, 503,000km, Old Man Emu suspension, rusty and running strong  Will certainly outlive all my other wheels
1985 Toyota 4wd pickup, single cab, rare 2lt 2.4l turbo diesel, 5 speed, custom flatdeck, timing belt every 100,000k and shes good to go. 393,000km Excellent fuel economy!
1983 Mercedes 300d, low mileage.....gorgeous!
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