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1982 GMC 2500 - switching to multi-grade oil

I have used regular 30 weight engine oil in my 6.2 diesel for many years. This oil is getting more difficult to find. Is there any problem with switching to a multi-grade oil?

No, and I’d recommend it. 15W40 grade diesel spec motor oil RotellaT4 or Delo or similar.

I see that RotellaT1 30W is still readily available as are other single weight oils. Why are you having problems finding it? Are you using 30W car oil instead of diesel oil?