Ok, I've done a lot of searching on this subject and don't see a lot of info specifically relating to the fact that the owners manual states to only use 5-30, or 0-30 for colder weather. I live in Las Vegas and the manual shows 5-30 all the way up past 100 degrees. Now I see that quite a few of you are using 10-40 and 15-40. Does is really make a difference, should I only use the 5-30 as stated in the manual or would you guys use the 10 or 15-40? I have 600 miles on the truck and want to do my first oil change. Any info would be appreciated. Thanks!

15w40 should be the recommended viscosity for Vegas. I don't see anything about 5-30 anywhere in my manual. Well that's not true, its in the standard manual but not in my Duramax Supplemental manual

OK, disregard my first post, I was reading in the stinkin' "gas engine" (normal thick) manual, not the Duramax supplemental manual. I see that 15-40 and other viscosities are acceptable. Thanks for the reply.

Not only are 15-40W or 5-40W acceptable, they're virtually the ONLY grades you should be using. The exception might be Amsoil 5-30W diesel oil.

Just make sure it's API CI-4 or preferably CI-4+ rated, and your Isuzu should be happy for years to come. :)

Not only are 15-40W or 5-40W acceptable, they're virtually the ONLY grades you should be using. The exception might be Amsoil 5-30W diesel oil.

Just make sure it's API CI-4 or preferably CI-4+ rated, and your Isuzu should be happy for years to come. :)

Look also for API CH-4 rating (low ash) together with API CI-4 to get the best of both worlds....Shell Rotella 15W40 from Wally World is cheap and just fine for that!