You can follow the DIC. I do the 2011 at around 7500 miles. We use cheap $7/gallon conventional oil but our UOA's always come back nice. The DIC is usually under 20% when I change it. The DIC max is `10,000 IIRC. 7500 is GMs reccomendation and is what the DEF is sized for.
The lube shop end of the dealership will always put 3,000 miles no matter what. That way they can have you back in every 3000 miles to find something else to fix. Its just that way. lube shops everywhere still do 3000 miles.
Anything under 5,000 to me is a waste with any oil. The Duramax is extremely easy on oil and is not picky.
I don't remember what spec but IIRC Delvac has been the factory fill.
My policy for determining OCI's for our fleet is this.
If it is under warranty, I go by the book. If the offer a high and a low "based on conditions" I pick the high and send a sample in. If something with their recommendation seems freakishly high I'll send a sample to confirm. The reason I do this for under warranty is due to the simple fact that if anything happens, I have less chance of the finger pointed at "improper maintenance" in their eyes.
If its not under any kind of warranty, then run it to the OEM's reccomendation and pull a sample, go longer, check again, and keep going until I find the limit. Then go to 3/4 of that limit and sample. If it is good, then thats what I use for the OCI and I will send one sample in annually for that machine. If an issue arises I will increase the UOAs until corrected.
Our feed truck is a good example. N14 Cummins. Cummins recommends 250hrs OCI. We put that on in a month, after some UOA's at intervals I've determined 1000hrs is a safe point for the cheap $7/gal oil we run for now.
With that truck, it costs about $110 to change the oil with filter and labor. I figure we'll probably get 5 years of use out of it. With a 250hr interval it would get changed 12 times per year, now its only getting changed 3 times. Thats a savings of $990. Over 5 years time that's just under $5,000.
Now lets look at the idea of "is it cheaper than an engine".
That engines useful life is generally 25,000hrs or so. For simple math we'll say it is.
That would mean you would change the oil 100 times. Lets say you use the $150 oil change special instead of doing it yourself for easy math. You would spend $15,000 on oil changes. You would save $11,250 by using a 1000hr interval. Now the big factor is will the engine wear out faster using the longer OCI. Who knows. If it makes it to 1000000 theres a lot of other factors playing into its wear.
