I think the additives in oils for diesels is probably better for the engine. As Larry alluded to they have a different additive package. Generally they have more additives and keep the engine cleaner. I did read a Car and Driver editorial that did mention that there is a chance these extra additives may harm the catalytic converter. I run Rotella in my gas motorhome (no converter) because it works especially hard. Otherwise I run Mobil 1 synthetics 15-50 summer and 10-30 winter.

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1995 G320
1984 280GE
1971 Unimog 416