Underpowered and oil may mean turbo or busted rings/piston. Whatever it is, it cannot be good. Sorry to hear that happened to you. I would do a compression test. If a cylinder is significantly low, do a leak down test. If you had to pull the exhaust manifold, a look at the exhaust ports would tell you which cylinder(s) was/were passing oil. Just a few thoughts.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi