The web is full of threads like the one I pasted above. I read a bunch of them before changing the oil in ours, and they all eventually lead to the same conclusion: that your Duramax isn't particular about what brand you use, as long as it's rated for diesels, and as long as it's a quality oil. Not some off-brand thing you found at Family Dollar on sale for two bucks. And that you'll likely get the same result with 5w-40 or 10w-40 as you would with 15w-40.
2006 Jayco Seneca 34SS
2006 Jeep Wrangler Unlimited Rubicon Toad