The difference is in the heat range -- I don't remember which was Bosch numbers go, hotter or colder, but each heat range has a different length and diameter of the ceramic insulator. This results in the tip of the sparkplug being hotter (longer, thinner insulator) or colder (shorter fatter insulator) for any given operating condition.

Stick with the standard W8 unless you have evidence of overheating that isn't related to lean running (fix the mixture control if it runs lean), or cold fouling (very unlikely).

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!