1984 cucv Engine oil type 6.2l diesel

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Thanks

But Delvac is not in not in 10w40.

Correct
It’s 15w-40

If you look at the regular 10w-40 on the back label, it will say it’s fine to use in diesels.

I use the Delvac in all my Diesels year around and have never had any issues.

Ironically, I use Rotella “diesel” motor oil in all my gasoline 4 cycle small engines.
Zero issues.