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I run delvac 1300 in mine. It's also excellent oil. I wouldn't run synthetic in a duramax myself. Not worth it.
Unless you're going to run extended oil change intervals,or live in the arctic don't waste your money on synthetic. The Duramax has proven to be very easy on oil. Run an appropriate conventional oil, follow the OLM and the engine will last longer than you'll probably want to drive it.