I've always just used rotella 15-40 in my diesel, and my father's diesels. Its good oil and we've never had any problems using it. I don't even switch to a lighter weight during the winter. of course, i do live in louisiana!

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1963 Ford F100 - Cummins Swap?
1985 Mercedes 300SD - 210K
1992 Ford Festiva - 92K
2001 Ninja 250 - 8K