what oil is best for my 1989 gmc sierra 2500 4 wd
What ever is recommended in your owners manual. Don't swap to synthetic with an engine that old. During summer months, it's fine to go to a heavier oil. Just be sure to go back to the recommended weight come winter.
Looking for a Used Sierra 1500 in your area?
CarGurus has 67,430 nationwide Sierra 1500 listings starting at $1,350.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts