Re: Synthetic Oil
You do not change synthetic oil every 3-5 thousand miles. When I used Amsoil, I changed it at 20,000 miles. I learned a long time ago I learned that oil does not break down. It gets dirty. IF you can keep oil clean, it will last a VERY long time. That makes the secret for long life, the filter. Amsoil has their filter for their oil. Back in 1998-2001, when I was using it in my Dodge, they had a 1996 Dodge truck that had over 130,000 miles on the base oil. It had their expensive, good filter system on it that I did not buy. They analyzed it every 20,000 miles, replaced the oil that got burned and kept going.
Read any manual that comes with the Duramax. It says under normal usage, you can go up to 10,000 miles with regular oil. The computer calculates the RPM's, temp., flow through the filter and monitors several other things the engine goes through and it will tell you when to change the oil with a display on the dashboard. At the change, you reset the computer for the next cycle. It also tells you when the level is low.
I know lots of people do not believe in going more than 3,000 miles on their oil. That is very old school these days. Engines are better, oils are better, filters are better. Read the owners manual that came with your vehicle. It will advise you best, but probably 5,000 miles or once a year for the average gas engine.