Use Mobil 1 or Mobil Delvac 1 only -- should last forever. I've read reports by an experienced tech that the 350 run on full synthetic lasts as long as the 3.0L 603, the ones that died were all run on dino oil.

I'd not do any swaping unless it bends a rod, no need to panic.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!