Low compression and sooty running will clog the rings -- getting the compression (leaky valves) fixed will raise combustion temps and clean up the top. so the rings get a chance to clear.

WVO is notorious for gumming up engines, it has too much protien in it.

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!