So do synthetic motor oils for our Duramax's only come in 5w 40 wt oil? I'm a little concerned in running 5w 40 when the owners manual recommends 15w40 oil.

It works good for me.
Plus turbo chargers like thin oil. They spool up faster and most of the oil thickeners coak faster then the rest of the oil.
After I changed the connecting rod and main bearing in my truck and switching to synthetic 5w-40 weight I have gained about 3mpg.
That puts me just under 20mpg.

I've alway run 5w30 synthetic oil in by gas powered Blazer and I swear by it but GM also recommends 5/30 so I didn't have a problem. But the Duramax recommends the 15w40 I'd sure hate to screw up an expensive diesel engine because of the oil.

Here a few 15W40 synthetic motor oils:
http://www.amsoil.com/storefront/ame.aspx
http://www.redlineoil.com/products_motoroil.asp?productID=7&subCategoryID=1&categoryID=1
http://www.synerlec.com/prodsa/rpmoa.html

royal purple to

i run the amsoil 5w-30 hdd diesel oil. i love it its great oil, smoothed out the engine and i saved money by extending the drain intervuls. as far as what the manual recomends, as long as the oil did not cause the problem the manufacturer has to warrenty the problem

it's fine. Performs as a 5w when cold and a 40 when hot. Once it's warm it will flow the same as the 15w40. Just better to flow like a 5w when it's cold and needs to flow. Your engine and turbo will thank you.