I just traded in my 2004 Silverado with a 6.0L engine and 140,000 miles. I used it to tow a 10,000# FW in the mountains of Arizona to Montana. I replaced the factory oil with Mobile 1 full synthetic at 200 miles, then used synthetic for every oil change since then. I changed the oil based upon the Oil Sensor system. When the light came on, I changed the oil.
At 125,000, I developed a small leak over one of the rocker arm covers. I removed the cover and replaced the gasket. What was amazing was that the rocker arms and top of the cylinder head look brand new! There was no sludge what-so-ever.
I would not say I got any better gas mileage than anyone else, but I felt much more confident in the longevity of my engine when using synthetic.
I also had a 1989 Mistubishi Montero that I'd purchased new. My wife and I commuted in it over 100 miles per day. When I traded it in several years later, I had 245,000 miles on the engine and had zero work done to it other than routine timing belts per the owners manual.
I know this is anecdotal evidence, and others probably have similar results with petroleum based oils, but there was no sense in me changing from what worked.
