Hustler, my understanding of GMs' oil change indicator is that the computer basically computes how hard the engine works and gives a change interval. There is not really a sensor that reads how "dirty" the oil is, its just a computer program that calculates how dirty it should be.
Hard towing takes more throttle and therfore you change the oil more often.
My wife has a 03 6.0L and I did the first couple of oil changes on it. She is one of those types that if the dealer said it that is the way it will be done. That being said I did the first oil change at like 7,500 miles, when the change oil light came on, and the oil was the worst that I had seen out of a new engine. But the dealer said that was when we should change the oil. I did the second, third and fourth oil changes when i felt they should be done, 3,000 to 3,500 miles. The oil was starting to look better after every oil change and I was happy. I quit doing the service on the truck, it's a company truck and it turned into more of a hassle than it was worth to me to change the oil.
She now takes it to the dealer when the truck tells her to change the oil. I checked the oil the other day before I drove the truck to go pick up a horse for her and was very unhappy with how dirty the oil was on the dip stick. I belive she has several thousand miles left to the next oil change light. I don't think that is a great thing for an engine with 40,000 miles on it.
That being said maybe the truck will run for hundreds of thousands of miles, I will never know because it will be traded in before that time. I don't feel that the truck will make much more than a hundred thousand miles before it starts to go down, the oil just looks to bad to me.
By the way, the filters on these trucks is little tiny compared to a 350 or 454. The diesel oil filter looks to be a 1 quart job. I know the 7.3 fords have a two quart filter on them. I feel that size does matter.
I don't mean for this to sound like a lecture or come off as a know it all, but I had to ask all of the right questions of the dealer. I also had to read the owners manuals very carefully to get the "truth" about how the computer knows when the oil is needing to be changed. The dealer made it sound exactly like you stated that there is an "oil contamination" sensor. My understanding is that there is an oil life program that takes input from several sources and computes the interval.
Take the above for what it's worth, some random person on the net with their opinion.
Doesn't sound very good, but then again I have no idea of thier background or if they are total rookies???
