Maybe the WVO is what made it run so long with out oil? that build up on the rings might have lubed it

I had the same power problem myself on my 300CD " same engine as your SD "
It was the Boost protection switch. It is on the driver side of the fire wall and has vac hoses running from the intake manifold and ALDA to it. When I replaced it the car was 4X faster. You should also have a look " and clean if needed " the banjo fittings for those lines. They can get blocked up and give you HUGE power loss.
Jay

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83 300TD Sable 402K " Work Truck "
81 300SD Gray 240K " Loner Car "
04 E320 Blue 90K " My Winter rig "
07 BMW X5 4.4 118K " My Summer rig"
04 Jetta TDI Wagon 168K " Wifes "
08 Toyota FJ 109K " Wifes winter rig "
04 Jetta TDI 120K " Daughters Car "

has the bowden cable been adjusted? my car had a tough time getting moving because it was constantly in top gear. i adjusted the cable, then POW! it's like a different car now.

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1982 300sd from craigslist $800 greased on one tank with NO CONVERSION in the Hot Texas Sun. (currently dead & awaiting engine damage investigation and/or longblock swap)
new daily: '03 vw 5speed jetta tdi wagon. bagged&chipped