Being we have a lot of farm equipment with diesel engines (some have Cummins) we buy oil in bulk and have went all to 5w-40 full synthetic. With oil sampling we have went to 500 hour oil changes and have yet to get back a bad sample. I use the same oil in my C7 Cat but change it once a year which is about 7000-8000 miles. I will be taking a sample this year. In my opinion one of the biggest advantage of synthetic oil is that at start up the engine get lubricated quicker. I have not noticed any notable increase in fuel economy. They say it is about 2% which would be very difficult to detect.:)
